DIN 3570 U-Bolt Dimensions and Size Chart

DIN 3570 U-bolt dimensions are tabulated by the pipe the bolt clamps, not by the thread. The size designation — 30, 38, 46, 52, 64, 82, 94, 120, 148, 176, 202, 228, 282, 332, 378, 428 and 530 — is the inner width a of the U in millimetres, and the standard pairs each of those sizes with a pipe outside diameter, a thread size on the legs, a minimum thread length, a leg centre distance and an overall height. Those are the 17 sizes in the standard’s own table, running from pipe outside diameter 25/26.9 mm up to 508/521 mm. The standard behind them is DIN 3570, dated October 1968, and the designation is still the one used to buy and sell the part.

The table on this page is compiled from the dimension table published with the standard, so you can look a size up by pipe diameter and by bolt size. It also carries size 23, which is not one of the standard’s sizes: several suppliers offer it as an extension below the standard’s range, and it is marked as such. Two things cause most wrong orders, and both are covered below: selecting by nominal pipe size instead of actual outside diameter, and trusting a DN printed next to a size number instead of the outside diameter the same table gives.

What DIN 3570 defines, and what it does not

DIN 3570 is titled Rundstahlbügel für Rohre — Steel strap for tubes of NW 20 to 500, in the English title recorded with the standard. It defines a round-bar U-bolt, bent from bar and threaded at both ends, supplied without nuts and without washers. Its scope is dimensional: the size, the pipe the size suits, the thread on the legs, the thread length, and the overall form. It is not a strength standard, a material standard or a coating standard, and it says nothing about working load.

That matters when you compare it with what the rest of the world calls the same part. DIN 3570 has no EN or ISO counterpart, and no ISO document replaces it. A German supplier dimension sheet for this product carries the note “entspricht keiner anderen Norm” — it corresponds to no other standard. The US pipe-hanger framework is a separate document with its own size designations and its own dimensions: MSS SP-58 covers U-bolts as one of its type designations, but DIN 3570 and MSS SP-58 are independent standards from different organisations, and a DIN 3570 size number does not carry across to an MSS type number automatically.

What the standard number on its own does not give you is thread size. The size designation encodes the inner width only — “A 46” says nothing about whether the legs are M10 or M12. For every size in the standard’s table, from 30 to 530, the thread is fixed by the table below, and it is not the number in the designation. If you have a DIN 3570 U-bolt in your hand or on a drawing and no table, measuring the inner width and the thread is the only way to place it.

The symbols on the DIN 3570 dimension drawing

Catalogue tables for this product disagree with each other mostly because they label the same drawing differently. These are the symbols as the standard’s own table uses them, and the labels that appear in their place elsewhere.

Two entries in that list are worth stopping on.

d2 is not the thread diameter. The bar the U-bolt is bent from measures less than the thread that is later rolled onto it. At M10 the bar is about 8.9 mm, at M12 about 10.7 mm, at M16 about 14.6 mm and at M20 about 18.2 mm. Each of those bar diameters sits about 2% below the pitch diameter of the matching metric coarse thread — 9.03 mm at M10, 10.86 mm at M12, 14.70 mm at M16, 18.38 mm at M20 — which is what a rolled thread formed on a bar of that size produces. A catalogue that lists one diameter per size and calls it “d” is usually giving you the thread, not the bar — and if you are checking that a U-bolt will pass through a hole or sit in a saddle, the two are not the same number.

The height column is not always the same dimension. The standard’s table carries h1, the overall height. It also carries h2 for the sizes up to 94, and published tables treat that second figure inconsistently: some print it, some leave it out, and some shift the column. Where h2 is given, it is not another way of stating the overall height — at size 30 the overall height is 70 mm and h2 is 28 mm. If overall height, rather than thread length, is the dimension the installation depends on, take it from the drawing the order is being made to instead of from a table whose “h” column is unlabelled.

Nominal pipe size is not the U-bolt size you need

This is the single most common error in specifying these bolts, and it is worth being blunt about it. DN is a reference designation, not a dimension. ISO 6708 defines DN as an alphanumeric size designation comprising the letters DN followed by a dimensionless whole number, indirectly related to the bore or the outside diameter of the pipe ends. It is not the outside diameter in millimetres, and it is not the U-bolt inner width either.

DN 25 pipe does not measure 25 mm across. Depending on which pipe standard it is made to, it measures 30.0 mm or 33.7 mm outside. A U-bolt with an inner width of 25 mm does not exist in DIN 3570, and would not fit either pipe if it did. The same gap runs the whole way up the range: DN 100 pipe is 108 mm or 114.3 mm outside, DN 250 is 267 mm or 273 mm.

There are two outside diameter series in common use in metric pipe, and DIN 3570 is built around both of them. The seamless steel pipe series under DIN EN 10220 gives smaller outside diameters for the same DN — 20.0 mm at DN 15, 25.0 mm at DN 20, 30.0 mm at DN 25, 38.0 mm at DN 32, 44.5 mm at DN 40. The threaded tube series under DIN EN 10255, and the ASME series, give the larger ones — 21.3 mm, 26.9 mm, 33.7 mm, 42.4 mm, 48.3 mm at the same five sizes.

DIN 3570 handles that by giving d1 as a range rather than a single value for the small sizes: size 30 is for pipe of 25 to 26.9 mm outside diameter, size 46 for 38 to 42.4 mm, size 120 for 108 to 114.3 mm. Both series fit the same bolt. Above size 94 the two series come close enough together that some sizes print a single figure.

So the working rule is this: measure the pipe, not the label on it. On refurbishment and spares work, the pipe in front of you is the only reliable input. Where you have to work from a DN, confirm the outside diameter from the pipe standard or the pipe marking before choosing the U-bolt, and check the wall of an old pipe where corrosion or a coating has changed its outside diameter. Also note that the DIN 3570 tables were drawn up for steel pipe; copper, stainless tube and plastic pipe of the same nominal size frequently have a different outside diameter, and the correct input is still the measured OD.

DIN 3570 size chart: sizes 30 to 94

All dimensions in millimetres, as published in the DIN 3570:1968-10 table. a is the inner width between the legs — the U-bolt size number. d1 is the outside diameter of the pipes the size is made for. b is the minimum thread length on each leg, e the distance between leg centres, h1 the overall height. The last column is the tabulated approximate mass per 1000 pieces, explained under the second table. The size 23 row is the supplier extension mentioned above, not a size the standard tables.

DIN 3570 size chart: sizes 120 to 530

Four things in those two tables are worth noting before you use them.

• The thread changes in steps, not with every size. Sizes 30 to 52 take M10; 64 to 94 take M12; 120 to 202 take M16; 228 to 332 take M20; and 378 and above take M24. There is no size in the standard’s table with a thread other than those five. The size 23 extension is also threaded M10.
• d2 is a tabulated standard dimension, not a manufacturer’s choice. The standard’s table gives the shank diameter d2 alongside the thread d3, and it is fixed for every size: about 8.9 mm at M10, 10.7 mm at M12, 14.6 mm at M16, 18.2 mm at M20 and 22 mm at M24. What a catalogue adds is the bar it actually draws, which is the same order of size but need not be identical to the finished shank.
• Size 23 is a supplier item, not a standard size. It is widely catalogued and easy to buy, but it sits below the range the standard covers. Its dimensions and the 68 in the weight column are supplier reference data, not DIN 3570 table values. Treat a size 23 requirement as a drawing-driven item.
• Size 202 sits on an ambiguous pipe diameter. The tabulated range is 191 to 193.7 mm. Some catalogues print it as 193.7 mm only, corresponding to DN 175, which is not a size that all pipe standards list. Check the actual pipe OD against both figures before ordering this one.

Form A and Form B

DIN 3570 defines two forms. Form A has two equal legs, both threaded, and is the form held in stock and the one the tables above describe. Form B has one long threaded leg and one shortened leg, for positions where the bolt can only be reached and tightened from one side; the shortened leg acts as the backstop. Because only one leg is threaded and fastened, Form B uses half the number of nuts — one per bolt up to size 64, two above it, as the mounting recommendation suppliers give, against two and four respectively for Form A.

Two practical consequences. Form B is available for the smaller sizes only — suppliers list it up to size 94, which is DN 80 — and it is normally a special order rather than a stocked item. And if a drawing specifies Form B while the quotation describes Form A, the parts are not interchangeable: the second leg is missing the thread and the height on that side is not h1.

Reference weights and what they are based on

The weight column in the tables above is the approximate mass published in the DIN 3570 table itself for the 17 standard sizes, in kilograms per 1000 pieces, calculated at a steel density of 7.85 kg/dm³. The 68 against size 23 is not one of those figures — that size is not in the standard’s table, so treat its weight as a supplier value. Read the basis before you use the numbers for freight or quoting.

• No nuts, no washers. The standard supplies the U-bolt only. If you need the assembled weight, add the nuts and washers separately — and if you need a weight for customs purposes, check what the commercial invoice is actually describing.
• Nominal geometry, not measured parts. The figures are consistent with a straight calculation from the tabulated dimensions at that density, so treat them as approximate. A checked weight on a delivered batch will differ slightly from a straight calculation, because bending, thread rolling and a commercial coating all move it a little.
• Stainless weights differ. The densities of austenitic stainless steels are not 7.85 kg/dm³, so the tabulated figures apply to steel. Stainless U-bolts of the same size weigh more, and a supplier quoting a stainless weight from a steel table is one to check.

Published catalogues disagree — the errors to check for

The tables published for this standard do not all agree, and one whole class of disagreement comes from something that is not in the standard at all. Working through the common versions, these are the faults that come up.

The DN printed next to a size number is a supplier convention. The standard’s table is indexed by pipe outside diameter; it does not assign a DN. Catalogues add one, so the same size can carry different DN labels — size 30 can be printed as DN 20, whose pipe is 25.0 mm outside in the seamless series and 26.9 mm in the threaded series, both inside the 25 to 26.9 mm range size 30 covers. Order on the outside diameter the table prints rather than on the label.

Size 23 treated as though it were a standard size. It appears at the top of many size lists because it is widely sold, but it is not in the standard’s table — one German supplier’s dimension table states plainly that DIN 3570 covers nominal widths 20 to 500 and that dimensions above and below that original range are manufacturer or supplier-programme variants. A table that presents it as the first DIN 3570 size implies the standard covers it and makes the whole ladder look one size longer than it is.

A DN column and an OD column from different pipe standards. A DN and an outside diameter can be individually correct and still be mismatched, because the same DN has two outside diameters. A table that labels its rows by DN and lists the DIN EN 10220 outside diameter while the size numbers came from the DIN EN 10255 series produces a table where every line is off by the difference between the two series — 3.7 mm at DN 25 and DN 32, 4.5 mm at DN 40 and DN 100.

One outside diameter where the standard gives a range. Printing 33.7 mm for the pipe a U-bolt takes, when the bolt actually covers 30 to 33.7 mm, makes a catalogue look tidier and hides the fact that the bolt also fits the seamless series.

Thread length five millimetres short at size 23. A majority of catalogues print b = 30 mm at size 23 and h = 60 mm. At least one large German trade catalogue prints 35 mm and 65 mm for the same size, and carries other small departures further up the table. Since size 23 is not in the standard’s table, neither set of figures can be checked against the standard — they are two suppliers’ versions of the same extension size, and the drawing decides. If a customer’s drawing quotes a thread length, check which table it came from rather than assuming the standard.

Thread described as the shank, or the shank as the thread. A row reading “M10 / 10” is a thread size and something else, not a thread size and a shank diameter — the shank at M10 is about 8.9 mm.

h2 dropped or relabelled. The second height dimension is absent from several otherwise complete tables, and where tables use “h” for the overall height and another column for a smaller figure, the smaller figure is not always h2.

A confident “this size = DN 20”. Any table that maps a single DIN 3570 size to a single DN is over-simplifying, because a DN does not point at one outside diameter — DN 20 pipe is 25.0 mm outside in the seamless series and 26.9 mm in the threaded series, which is why size 30 covers both. Cross-check every such mapping against the outside diameter.

Checking a DIN 3570 U-bolt before you order

1. Measure the pipe outside diameter, not the nominal size. Calipers on the pipe in front of you. Where the pipe is not accessible, get the outside diameter from the pipe standard or the pipe’s own marking, and state which series applies.
2. Find the size whose d1 covers that diameter. Size 46 covers 38 to 42.4 mm, so it takes DN 32 seamless at 38 mm and DN 32 threaded tube at 42.4 mm. If the measured diameter sits between two sizes, it belongs to the size whose range contains it — and a badly corroded or heavily coated pipe that has grown past its size range is a different problem, not a reason to jump a size.
3. Read the thread off the table, not off the size number. Size 94 is M12 and the next size up, 120, is M16. Both the diameter step and the thread step sit between those two sizes, so the a value alone will not tell you which thread you are ordering.
4. Check the thread length against the support. b is a minimum thread length on each leg. If the bracket, beam or channel is thicker than that leaves, the bolt is the wrong one, and a longer-thread or longer-leg variant is a different product from the standard item.
5. Check the overall height against what is above the pipe. h1 runs from 70 mm at size 30, the smallest size in the standard’s table, to 589 mm at size 530. Overall height is what usually rules a size out in a congested ceiling or a duct. If there is a reason to suspect your catalogue’s height column, ask for the dimension from the drawing.
6. Decide Form A or Form B from the access, and say which. Two equal threaded legs, or one long leg and one short. This is the point at which a quotation most often comes back with the wrong part.
7. Specify material, grade and finish. DIN 3570 does not. Steel U-bolts are commonly supplied in S235 or in strength grades such as 8.8, and stainless in A2 or A4; finishes range from plain to zinc plated, hot dip galvanised and zinc flake. Leave these out and you will get whichever is the supplier’s default.
8. State whether nuts and washers are included. The standard supplies the bolt alone. Nuts and washers are normally ordered separately, in the size the thread calls for.

How to write the designation

Trade and supplier designations for this product follow the same pattern: the standard, the material or finish code, the size, the pipe size it is for, and the thread. Published catalogue lines look like this —

DIN 3570 – St – A 46 / NW 32 / M10 – verzinkt

DIN 3570 – Zn – A46 – DN 32 – M10

— where St or Zn distinguishes steel from galvanised, and NW 32 and DN 32 both point at the same pipe size. Either convention is understood. What has to be present for the order to be unambiguous is the standard, the size letter and number, the thread, the material or grade, and the finish. Add the pipe size you are clamping when the bolt is being bought for a known line, and add the overall height or thread length when either is critical, because those are the dimensions most often quoted differently between catalogues. A full line for a drawing or a purchase order therefore reads:

U-bolt DIN 3570 / Form A / size 120 / M16 / S235JR / hot dip galvanised / overall height 171 mm / nuts and washers excluded

Where DIN 3570 stands as a standard today

DIN 3570 was first published in 1963 and the current edition is dated October 1968, which replaced the 1963 edition, according to the standard’s catalogue record. It carries no EN or ISO successor and no companion document covering the same product in the European framework. Its cross-references in that record point sideways rather than forwards — to DIN 267-4 for technical delivery conditions, DIN 1013 for hot-rolled round steel and DIN 17100 for general structural steels — which is consistent with a dimensional standard that leaves material and performance to other documents and assumes the national steel grades of its time.

Two practical readings follow. First, the designation is safe to buy against: it is still the name under which the product is catalogued, quoted and shipped across Europe, and manufacturers in several countries still produce to it. Second, the standard is old enough that the names it references — St 37 and the DIN 17100 grades — are no longer the grades a mill will quote. When a drawing calls up a DIN 3570 U-bolt and names a material from the standard’s own era, expect to agree the current equivalent grade with the supplier rather than finding it in the standard.

The short version

DIN 3570 U-bolt dimensions are keyed to the pipe outside diameter, in 17 standard sizes from 30 to 530, with the thread stepping through M10, M12, M16, M20 and M24 as the size grows. Some suppliers also offer a size 23 below the standard’s range, which is not one of the 17. The size number is the inner width of the U — not the pipe size, not the thread size, and not a DN. Select by measured outside diameter, read the thread off the table rather than off the size number, treat DN as a label rather than a dimension, and check that the DN a catalogue prints matches the outside diameter it gives. The standard supplies a bare U-bolt without nuts or washers, and it does not cover material, grade, coating or working load, so all four have to be stated separately.

ChinaBolts supplies DIN 3570 U-bolts in Form A and Form B, in carbon and stainless steel, across the size range above, as part of a wider bolts range that includes the hex products in the same standards framework. Send the pipe outside diameter, the size and thread, or just the pipe you are clamping with its standard and OD, and we will confirm the size, the thread and the overall height against the table before the order is placed — tell us what you need.