Bolt Length Chart: Pick the Right Bolt Length for the Job

What does a 2-inch bolt at 50.80 mm mean?

On the default — a 2-inch bolt — the millimetres are 50.80 mm and the class is standard. The millimetres are the inches times 25.4: 2 × 25.4 = 50.80 mm. A 2-inch bolt is the general-purpose standard — long enough to pass through the pieces and take a nut with thread to spare, short enough not to stick out and snag. Whether that is the right bolt depends on the joint — a softwood joint sizes up one row, and a thin joint sizes down.

Three things about the model are worth stating plainly.

First, the conversion is 2 × 25.4 = 50.80 mm, and the class is standard — the millimetres and the class are two outputs, never the same number. Second, the meaning is 50.80 mm of bolt for a general-purpose joint — the workshop’s bolt, the middle of the 1.25–2.5 in standard zone, not the size of the joint itself. And third, a length between rows sizes UP: 1.75 in converts directly to 44.45 mm, but the chart gives the 2-in row at 50.80 mm — standard — because a bolt that is too short will not take a nut and a bolt that is too long still holds — all listed in Accuracy & limitations.

The worked example, line by line

Setup: bolt 2 in.

Line 1 — the millimetres. 2 × 25.4 = 50.80 mm.

Line 2 — the class. 2 in sits in the 1.25–2.5 in standard zone — standard.

Line 3 — the check. At 1.75 in the direct conversion is 44.45 mm, which sizes up to the 2-in row at 50.80 mm — standard.

Every bolt length in the table, checked

The table runs from 0.5 to 6 inches. Every length in the table sits inside one of the four classes: 0.5–1 in are short bolts, 1.25–2.5 in are standard bolts, 3–4 in are long bolts and 5–6 in are extra-long bolts. The classes are closed at both ends — 1.25 in is a standard bolt, 3 in is a long bolt and 6 in is an extra-long bolt. Lengths below 0.5 in and above 6 in match NO standard class — below 0.5 in the fastener is a machine screw sized by the maker; above 6 in the bolt is a through-bolt or anchor bolt sized by the engineer. The table is a synthesis of published fastener guidance for household use — a 2-in bolt for a general-purpose joint is the published standard, and the length roughly doubles as the joint grows. Bolt length varies by material, load and use — size up one row for softwood or a joint that will be taken apart often, and always check the fastener maker’s own table for the exact product.

No bolt-length page of any kind exists on this site — the closest page is the bolt-size-chart page, which covers bolt diameter and threads-per-inch and says in its own FAQ that “the length is measured separately” — it does NOT cover length. This page is the bolt length’s own size chart: length in inches and millimetres, by class, with a live lookup that sizes up to the next row.

0.5 in: 0.5 in = 12.70 mm; short — The bottom of the table, a short bolt.

0.75 in: 0.75 in = 19.05 mm; short — A short bolt, one row above the bottom.

1 in: 1 in = 25.40 mm; short — A short bolt, the top of the short zone.

1.25 in: 1.25 in = 31.75 mm; standard — The bottom of the standard zone, a standard bolt.

1.5 in: 1.5 in = 38.10 mm; standard — A standard bolt, one row below the middle.

2 in: 2 in = 50.80 mm; standard — The general-purpose standard, the middle of the standard zone.

2.5 in: 2.5 in = 63.50 mm; standard — A standard bolt, the top of the standard zone.

3 in: 3 in = 76.20 mm; long — A long bolt, the bottom of the long zone.

3.5 in: 3.5 in = 88.90 mm; long — A long bolt, the middle of the long zone.

4 in: 4 in = 101.60 mm; long — A long bolt, the top of the long zone.

5 in: 5 in = 127.00 mm; extra-long — An extra-long bolt, the bottom of the extra-long zone.

6 in: 6 in = 152.40 mm; extra-long — The top of the table, an extra-long bolt.

The reverses, worked backwards

Smallest table row whose width reaches 50.00 mm: At the displayed 2-in row the figures are 2 in = 50.80 mm; standard. At the 1.5-in row the figures are 1.5 in = 38.10 mm; standard — below 50.00 mm — so the 2-in row is the smallest table row whose width reaches 50.00 mm.

Largest table row whose width STAYS at or below 50.00 mm: At the displayed 1.5-in row the figures are 1.5 in = 38.10 mm; standard. At the 2-in row the figures are 2 in = 50.80 mm; standard — above 50.00 mm — so the 1.5-in row is the largest table row whose width STAYS at or below 50.00 mm.

Smallest table row that needs a standard bolt: At the displayed 1.25-in row the figures are 1.25 in = 31.75 mm; standard. At the 1-in row the figures are 1 in = 25.40 mm; short — short — so the 1.25-in row is the smallest table row that needs a standard bolt.

Smallest table row whose width reaches 100.00 mm: At the displayed 4-in row the figures are 4 in = 101.60 mm; long. At the 3.5-in row the figures are 3.5 in = 88.90 mm; long — below 100.00 mm — so the 4-in row is the smallest table row whose width reaches 100.00 mm.

Largest table row whose width STAYS at or below 100.00 mm: At the displayed 3.5-in row the figures are 3.5 in = 88.90 mm; long. At the 4-in row the figures are 4 in = 101.60 mm; long — above 100.00 mm — so the 3.5-in row is the largest table row whose width STAYS at or below 100.00 mm.

Smallest table row that needs an extra-long bolt: At the displayed 5-in row the figures are 5 in = 127.00 mm; extra-long. At the 4-in row the figures are 4 in = 101.60 mm; long — long — so the 5-in row is the smallest table row that needs an extra-long bolt.

What the defaults assume

The defaults are a 2-inch bolt — 2 in = 50.80 mm; standard. With 0.5 in, 0.5 in = 12.70 mm; short. With 0.75 in, 0.75 in = 19.05 mm; short. With 1 in, 1 in = 25.40 mm; short. With 1.25 in, 1.25 in = 31.75 mm; standard. With 1.5 in, 1.5 in = 38.10 mm; standard. With 2 in, 2 in = 50.80 mm; standard. With 2.5 in, 3 in, 3.5 in, 4 in, 5 in, 6 in, 2.5 in: 63.50 mm — standard; 3 in: 76.20 mm — long; 3.5 in: 88.90 mm — long; 4 in: 101.60 mm — long; 5 in: 127.00 mm — extra-long; 6 in: 152.40 mm — extra-long. With Width 0, 0 in = 0.00 mm; matches no standard class — no bolt at all — 0.00 mm and no class. With Width -0.5, 0 in = 0.00 mm; matches no standard class — a negative length is floored to 0.00 mm — no negative bolt. With Width 1.75 in, 1.75 in = 50.80 mm (direct 44.45 mm — sizes up to the next row); standard — the direct conversion is 44.45 mm — it sizes up to the next row. With Width 8 in, 8 in = 152.40 mm; extra-long — past the top of the table — clamped to the 6-in row with the note to size by the fastener maker’s table. With Width 0.25 in, 0.25 in = 12.70 mm; matches no standard class — below the shortest bolt on this chart — clamped to the 0.5-in row with NO class; the direct conversion is 6.35 mm. With Width 2.75 in, 2.75 in = 76.20 mm (direct 69.85 mm — sizes up to the next row); long — the direct conversion is 69.85 mm — it sizes up to the next row. With Width 1 in, 1 in = 25.40 mm; short — exactly on the row — the bottom of the short zone. With Width 4.25 in, 4.25 in = 127.00 mm (direct 107.95 mm — sizes up to the next row); extra-long — the direct conversion is 107.95 mm — it sizes up to the next row. With Width 3.75 in, 3.75 in = 101.60 mm (direct 95.25 mm — sizes up to the next row); long — the direct conversion is 95.25 mm — it sizes up to the next row. With Width 5.5 in, 5.5 in = 152.40 mm (direct 139.70 mm — sizes up to the next row); extra-long — the direct conversion is 139.70 mm — it sizes up to the next row. With Width 0.05 in, 0.05 in = 12.70 mm; matches no standard class — below the shortest bolt on this chart — clamped to the 0.5-in row with NO class; the direct conversion is 1.27 mm. With Width 25.4 in, 25.4 in = 152.40 mm; extra-long — past the top of the table — clamped to the 6-in row with the note to size by the fastener maker’s table.

A note on the rounding: the millimetres are displayed to two decimals, half-up. All twelve table rows convert to exact one-decimal millimetres, so the table itself carries no rounding trap — the trap lives in the size-up edges, where a between-rows length converts to a fractional millimetre. At 1.75 in the direct conversion is 44.45 mm — it sizes up to the 2-in row at 50.80 mm. At 2.75 in the direct conversion is 69.85 mm — it sizes up to the 3-in row at 76.20 mm. At 4.25 in the direct conversion is 107.95 mm — it sizes up to the 5-in row at 127.00 mm. At 3.75 in the direct conversion is 95.25 mm — it sizes up to the 4-in row at 101.60 mm. At 5.5 in the direct conversion is 139.70 mm — it sizes up to the 6-in row at 152.40 mm. The page prints the direct conversions so the sizing up does not mislead you.

Output · formula · meaning

Who uses this

• The homeowner bolting a gate hinge: they learn a 2-in bolt is 50.80 mm — standard — and they check the 4-in row to see the first row at or above 100.00 mm.
• The renter fixing a wobbly chair: they learn a 0.5-in bolt is 12.70 mm — short — and they check the 1.25-in row to see the first row needing a standard bolt.
• The contractor through-bolting a beam: they learn a 5-in bolt is 127.00 mm — extra-long — and they check the 3.5-in row to see the last row at or below 100.00 mm.
• The student learning fasteners: they learn the bolt length is the inches times 25.4 — and they check the 5-in row to see the first row needing an extra-long bolt.

Accuracy & limitations

The math is first-principles: the millimetres are the bolt length in inches times 25.4, and the class comes from the sized-up table row — all in inches and millimetres, the millimetres displayed to two decimals half-up. There is NO division in this calculator — nothing can be undefined — but there IS sizing up: a 1.75-in length converts directly to 44.45 mm, but the chart gives the 2-in row at 50.80 mm — standard — because a bolt that is too short will not take a nut. A 2.75-in length converts directly to 69.85 mm, but the chart gives the 3-in row at 76.20 mm — long. A 4.25-in length converts directly to 107.95 mm, but the chart gives the 5-in row at 127.00 mm — extra-long. For lengths above 6 in the core clamps to the 6-in row (152.40 mm) and reports the extra-long class with a note to size by the fastener maker’s table; for lengths below 0.5 in (but above 0) it clamps to the 0.5-in row (12.70 mm) and reports NO class. At 0 in the figure is 0.00 mm — a real figure, not undefined — and a negative length is floored to 0.00 mm. The classes are closed at both ends: short 0.5–1 in, standard 1.25–2.5 in, long 3–4 in, extra-long 5–6 in — so 1.25 in is a standard bolt, 3 in is a long bolt and 6 in is an extra-long bolt. Bolt length is measured under the head to the tip — the head is not part of the length, and the threaded portion is counted within it. A bolt that “looks longer than its size” is usually being measured with the head included. The table is a synthesis of published fastener guidance for household use — a 2-in bolt for a general-purpose joint is the published standard, and the length roughly doubles as the joint grows. Bolt length varies by material, load and use — size up one row for softwood or a joint that will be taken apart often, and always check the fastener maker’s own table for the exact product. What the model cannot do is say whether the bolt will hold your joint: it says nothing about the material — whether the joint is hardwood or softwood — and it says nothing about the load — whether the joint carries weight or just holds two pieces together. Use it to pick the length — then check the bolt-size chart, screw size chart and nut size chart before you buy the hardware.

Frequently asked questions