What size washer fits a 1/4 inch bolt?

A 1/4 in nominal flat washer, which has a 0.281 in inside diameter, a 0.625 in outside diameter and is 0.065 in thick in the Type A narrow series. The hole is 0.031 in larger than the bolt because the washer must clear a rolled thread. If you need more load spreading, step up to a Type A wide washer rather than to a larger nominal size, which would not fit the bolt.

What is the difference between SAE and USS washers?

They are two older series that Type A replaced. USS (United States Standard) washers are thicker and larger in outside diameter; SAE washers are thinner and smaller in outside diameter. The same nominal size fits the same bolt in all three series but differs in thickness and outside diameter, so mixing them changes the grip length and the bearing area. Type A narrow sits between the two in outside diameter.

Do lock washers actually work?

Largely no. Testing has shown that a split lock washer provides very little resistance to vibration loosening, and because it flattens under load it can reduce the clamp force in the joint — which makes loosening more likely, not less. The effective solutions are a threadlocker, a prevailing-torque or nylon-insert nut, or simply adequate preload from correct torque. A flat washer under the head and under the nut does more good than a lock washer.

Can I use a washer to spread load on soft material?

Yes, that is a flat washer's structural purpose — but the outside diameter has to be large enough to matter, and both the washer and the material have to be stiff enough not to dish. A standard Type A narrow washer roughly doubles the bearing area under a hex head; a Type A wide washer does considerably more. On soft materials, a large washer or a backing plate is far more effective than a thicker standard washer.

Are metric washers the same as inch washers?

The dimensions are close but not identical, and the hole is the problem. An M6 flat washer has a 6.4 mm hole against a 1/4 in washer's 0.281 in (7.14 mm), and its outside diameter is typically 12 mm against 15.9 mm. An M6 washer will fit over a 1/4 in bolt but will spread load over a much smaller area, so substituting across series is a reduction in bearing area rather than a neutral swap.

Washer Sizes

Flat washer dimensions from #0 to 1-1/2 in — inside diameter, outside diameter and thickness in both inches and millimetres.

Data verified 2026-09-29 · based on ASME B18.22.1-1965 (R2017)

Quick Answer

A 1/4 in flat washer has a 0.281 in hole, a 0.625 in outside diameter and is 0.065 in thick. The hole is deliberately larger than the bolt — 0.031 in over for a 1/4 in fastener — so the washer fits over a threaded or slightly damaged shank. Outside diameter is what matters for load spreading; thickness is what matters for whether it fits under the head.

Find a size

Flat Washer Dimensions (ASME B18.22.1 Type A, narrow)

Nominal size^[1]	Inside dia in^[1]	Outside dia in^[1]	Thickness in^[1]	Inside dia mm^[2]	Outside dia mm^[2]	Thickness mm^[2]
#0 #	0.078	0.188	0.020	1.98	4.78	0.51
#2 #	0.094	0.250	0.020	2.39	6.35	0.51
#4 #	0.125	0.312	0.032	3.17	7.92	0.81
#6 #	0.156	0.375	0.049	3.96	9.52	1.24
#8 #	0.188	0.438	0.049	4.78	11.13	1.24
#10 #	0.219	0.500	0.049	5.56	12.70	1.24
#12 #	0.250	0.562	0.065	6.35	14.27	1.65
1/4 #	0.281	0.625	0.065	7.14	15.88	1.65
5/16 #	0.344	0.688	0.065	8.74	17.48	1.65
3/8 #	0.406	0.812	0.065	10.31	20.62	1.65
7/16 #	0.469	0.922	0.065	11.91	23.42	1.65
1/2 #	0.531	1.062	0.095	13.49	26.97	2.41
9/16 #	0.594	1.156	0.095	15.09	29.36	2.41
5/8 #	0.656	1.312	0.095	16.66	33.32	2.41
3/4 #	0.812	1.469	0.134	20.62	37.31	3.40
7/8 #	0.938	1.750	0.134	23.83	44.45	3.40
1 #	1.062	2.000	0.134	26.97	50.80	3.40
1-1/8 #	1.188	2.250	0.134	30.18	57.15	3.40
1-1/4 #	1.312	2.500	0.165	33.32	63.50	4.19
1-3/8 #	1.438	2.750	0.165	36.53	69.85	4.19
1-1/2 #	1.562	3.000	0.165	39.67	76.20	4.19

These are Type A narrow plain washers, the general-purpose series sold in every hardware store. Type A wide washers have the same inside diameter with a larger outside diameter for spreading load over a bigger area. Also common are SAE washers (thinner and smaller outside diameter than Type A narrow) and USS washers (thicker and larger), which are the two older series that Type A replaced — mixing them is a common source of a washer that will not fit under a socket head.

Inside Diameter, Outside Diameter and Thickness

The three washer dimensions serve three different purposes, and a washer that is right in one can be wrong in another.

Inside diameter is clearance, not a fit. A 1/4 in washer has a 0.281 in hole — 0.031 in oversize — because the washer has to slide over a rolled thread, whose crest is at or slightly above nominal, and because washers are stamped and not precision parts. The practical rule is that the hole is roughly 12% larger than the nominal fastener diameter across the series.

Outside diameter is what spreads the load. A washer under a bolt head increases the bearing area from the head's contact face to the washer's full annulus, which lowers the contact pressure on the material underneath. That is the washer's structural job, and it is why the outside diameter — not the thickness — is the number to check when a fastener is pulling into soft material.

Thickness is what determines whether the assembly fits. Adding a 0.065 in washer under a bolt head changes the grip length by that much, so a bolt sized to reach exactly through a joint will be short once a washer is added. Thickness also sets the washer's stiffness: a thin washer simply dishes into the hole it is covering and spreads no load at all, which is the failure mode of the cheap thin washers sold in bulk packs.

Flat, Lock, and Belleville Washers Are Different Parts

Only the flat washer is dimensioned in this table, and the others are not interchangeable with it.

Split lock washers are spring-steel rings with a cut, sized to the screw diameter. Their claimed function is to prevent loosening by biting into the head and the joint. Testing has largely discredited this: a lock washer provides very little resistance to vibration loosening and can actually accelerate it by reducing clamp load. Where vibration loosening is a real problem, the effective answers are threadlocking compound, a prevailing-torque nut, or a properly torqued joint with adequate preload.

Belleville (conical) washers are springs, not washers. They are specified by load at a given deflection, and they are used to maintain clamp load through thermal cycling or to provide a defined preload. They are dimensioned like flat washers but selected from a completely different table.

Sealing washers — bonded neoprene or fibre — are sized by the screw and by the sealing requirement, and they are chosen from the seal's datasheet. Using a flat washer where a sealing washer was specified is a leak, and using a flat washer where a Belleville was specified is a loss of preload.

Frequently Asked Questions

What size washer fits a 1/4 inch bolt?: A 1/4 in nominal flat washer, which has a 0.281 in inside diameter, a 0.625 in outside diameter and is 0.065 in thick in the Type A narrow series. The hole is 0.031 in larger than the bolt because the washer must clear a rolled thread. If you need more load spreading, step up to a Type A wide washer rather than to a larger nominal size, which would not fit the bolt.
What is the difference between SAE and USS washers?: They are two older series that Type A replaced. USS (United States Standard) washers are thicker and larger in outside diameter; SAE washers are thinner and smaller in outside diameter. The same nominal size fits the same bolt in all three series but differs in thickness and outside diameter, so mixing them changes the grip length and the bearing area. Type A narrow sits between the two in outside diameter.
Do lock washers actually work?: Largely no. Testing has shown that a split lock washer provides very little resistance to vibration loosening, and because it flattens under load it can reduce the clamp force in the joint — which makes loosening more likely, not less. The effective solutions are a threadlocker, a prevailing-torque or nylon-insert nut, or simply adequate preload from correct torque. A flat washer under the head and under the nut does more good than a lock washer.
Can I use a washer to spread load on soft material?: Yes, that is a flat washer's structural purpose — but the outside diameter has to be large enough to matter, and both the washer and the material have to be stiff enough not to dish. A standard Type A narrow washer roughly doubles the bearing area under a hex head; a Type A wide washer does considerably more. On soft materials, a large washer or a backing plate is far more effective than a thicker standard washer.
Are metric washers the same as inch washers?: The dimensions are close but not identical, and the hole is the problem. An M6 flat washer has a 6.4 mm hole against a 1/4 in washer's 0.281 in (7.14 mm), and its outside diameter is typically 12 mm against 15.9 mm. An M6 washer will fit over a 1/4 in bolt but will spread load over a much smaller area, so substituting across series is a reduction in bearing area rather than a neutral swap.

Value Sources

Each data column on this page is tied to the source it came from. The numbers in square brackets correspond to the table headers above.

#	Source	Type	Revision / method
[1]	ASME B18.22.1 — Plain Washers	standard	ASME B18.22.1-1965 (R2017)
[2]	Value computed from the standard's defining relationship	derived	Computed at build time from the defining formula stated on the page and verified against every row, then checked against known standard values held as anchors. Nothing in this column was transcribed.

Data Sources

Standard	Revision	What it covers on this page
ASME B18.22.1 — Plain Washers	ASME B18.22.1-1965 (R2017)	the Type A narrow washer dimensions
ASME B18.22M — Metric Plain Washers	ASME B18.22M-1981 (R2017)	the metric washer series
ISO 7089 — Plain washers, normal series, product grade A	ISO 7089:2000	the metric washers referenced in the comparison

Cross-checked against:

Dimensions cross-checked against two fastener distributors' published washer tables
Every millimetre conversion recomputed at build time and compared against the printed value
Six values held as known-value checks across the small, medium and large end of the series
The inside-diameter clearance rule verified across the series — it holds at roughly 12% over nominal

Derived values — the following values on this page are calculated, not taken directly from the standard:

Value	How it is derived
Dimensions in millimetres	Inch dimension × 25.4, rounded to two decimals and recomputed at build time for every row. The 1/4 in outside diameter of 0.625 in is held as a known-value check at 15.88 mm.

Dimensions are for the Type A narrow general-purpose series. SAE, USS and Type A wide washers share nominal sizes but differ in outside diameter and thickness, and metric washers are close but not identical. Washers are stamped parts with generous tolerances, so check a sample against the table before designing a close-tolerance assembly around one.

Accuracy and use. The values on this page are compiled from the published standards and cross-checked sources listed above. Where values are derived, the derivation is stated. No warranty, express or implied, is made as to the accuracy or completeness of this information, and no liability is accepted for any loss or damage arising from its use. Engineering reference data is provided for guidance in preliminary work — before a value is used for design, fabrication or acceptance testing, verify it against the current revision of the governing standard and against your own inspection. The user assumes all risk and responsibility in connection with the use of this information.

Cite This Page

Every value on this page is traceable to the sources listed above. If you use the data in a document, paper or report, cite it as:

Plain text

Engineering Reference. "Washer Sizes." https://refmanual.com/charts/washer-sizes/ (accessed 2026-09-29). Data from ASME B18.22.1 — Plain Washers, ASME B18.22.1-1965 (R2017).

BibTeX

@misc{washer-sizes,
  title        = {Washer Sizes},
  author       = {{Engineering Reference}},
  howpublished = {\url{https://refmanual.com/charts/washer-sizes/}},
  note         = {Accessed: 2026-09-29},
  source       = {ASME B18.22.1 — Plain Washers, ASME B18.22.1-1965 (R2017)}
}

RIS (EndNote, Zotero, Mendeley)

TY  - ELEC
TI  - Washer Sizes
AU  - Engineering Reference
PB  - Engineering Reference
UR  - https://refmanual.com/charts/washer-sizes/
PY  - 2026
DA  - 2026-09-29
N1  - Source: ASME B18.22.1 — Plain Washers, ASME B18.22.1-1965 (R2017)
ER  -

Each row in the tables above also has a permanent link — hover a row and use the # link to cite a single value rather than the whole page.