Rope Diameter Chart: Pick the Right Rope for the Job

What does a 1/2-inch rope at 12.70 mm mean?

On the default — a 1/2-inch rope — the diameter is 12.70 mm and the class is medium duty. The diameter is the thickness of the rope itself, and 1/2 in is the standard rope for a cruising boat — the general-purpose dock-line and anchor-rode size, strong enough to hold in a blow without being too stiff to handle the way a 1-in rope is. The millimetres are the inches times 25.4: 0.5 × 25.4 = 12.70 mm. The classes are CLOSED at both ends: light 1/4–3/8 in, medium 1/2–3/4 in, heavy 7/8–1-1/2 in (rope DIAMETER). Diameters between the classes (3/8–1/2 in and 3/4–7/8 in) match NO standard class — the classes have gaps, they are not continuous — and diameters of 1-1/2 in and above, and below 1/4 in, match NO standard class either.

Three things about the model are worth stating plainly.

First, the conversion is 0.5 × 25.4 = 12.70 mm. Second, the meaning is a 1/2-inch rope for a 30–40 ft boat — the general-purpose dock-line and anchor-rode size, strong enough to hold in a blow without being too stiff to handle. And third, the millimetre figure is displayed to two decimals, half-up, and a figure that displays as 12.70 mm can sit a hair below the line — 1/2 in gives 12.7000 mm, which displays as 12.70 mm — and one that displays as 9.53 mm can sit a hair below it — 3/8 in gives 9.5250 mm, which displays as 9.53 mm. The page prints the unrounded value so the rounding does not mislead you.

The worked example, line by line

Setup: rope 1/2 inch in diameter.

Line 1 — the millimetres. 0.5 × 25.4 = 12.70 mm (unrounded 12.7000 mm).

Line 2 — the class. 1/2 inch sits in the middle of the 1/2–3/4 in medium class, so the class is medium duty.

Line 3 — the check. At the 1/2-in row the diameter is 12.70 mm, above 10.00 mm; and at the 3/8-in row it is 9.53 mm — below 10.00 mm.

Every rope diameter in the table, checked

The table runs from 1/4 to 1-1/2 inches. Every diameter in the table sits inside one of the three classes: 1/4–3/8 in are light ropes, 1/2–3/4 in are medium ropes and 7/8–1-1/2 in are heavy ropes. The classes are closed at both ends — 3/8 in is a light rope, 3/4 in is a medium rope and 1-1/2 in is a heavy rope. Diameters between the classes (3/8–1/2 in and 3/4–7/8 in) match NO standard class, and diameters of 1-1/2 in and above, and below 1/4 in, match NO standard class either. The table is a synthesis of published rope-sizing guidance for moderate conditions — a 1/2-in rope for a 30–40 ft boat is the published standard, and the diameter roughly doubles as boat size grows. Sizing varies by rope material (nylon stretches and absorbs shock, polyester holds its length, polypropylene floats but weakens in sun), construction, and load — size up one row for exposed moorings or storm conditions, and always check the rope maker's own table for the exact product.

1/4 in: 1/4 in = 6.35 mm; light duty — 1/4 inch is the bottom of the table, a light rope.

5/16 in: 5/16 in = 7.94 mm; light duty — 5/16 inch is a light rope, one row above the bottom of the table.

3/8 in: 3/8 in = 9.53 mm; light duty — 3/8 inch is a light rope, the top of the light class.

1/2 in: 1/2 in = 12.70 mm; medium duty — 1/2 inch is a medium rope, the bottom of the medium class.

5/8 in: 5/8 in = 15.88 mm; medium duty — 5/8 inch is a medium rope, the middle of the medium class.

3/4 in: 3/4 in = 19.05 mm; medium duty — 3/4 inch is a medium rope, the top of the medium class.

Diameter rows — 1/4 in, 1/2 in, 1-1/2 in: 1/4 in: 6.35 mm — light duty; 1/2 in: 12.70 mm — medium duty; 1-1/2 in: 38.10 mm — heavy duty — one diameter from each class: the bottom of the table, the classic cruising-boat rope and the top of the table.

The reverses, worked backwards

Smallest table row whose diameter reaches 10.00 mm: At the displayed 1/2-in row the figures are 1/2 in = 12.70 mm (unrounded 12.7000 mm); medium duty. At the 3/8-in row the diameter is 9.53 mm (unrounded 9.5250 mm), below 10.00 mm — so the 1/2-in row is the smallest table row whose diameter reaches 10.00 mm.

Largest table row whose diameter STAYS at or below 10.00 mm: At the displayed 3/8-in row the figures are 3/8 in = 9.53 mm (unrounded 9.5250 mm); light duty. At the 1/2-in row the diameter is 12.70 mm (unrounded 12.7000 mm), above 10.00 mm — so the 3/8-in row is the largest table row whose diameter stays at or below 10.00 mm.

Smallest table row that needs a medium rope: At the displayed 1/2-in row the figures are 1/2 in = 12.70 mm (unrounded 12.7000 mm); medium duty. At the 3/8-in row the diameter is 9.53 mm, a light rope — so the 1/2-in row is the smallest table row that needs a medium rope.

Smallest table row whose diameter reaches 20.00 mm: At the displayed 7/8-in row the figures are 7/8 in = 22.23 mm (unrounded 22.2250 mm); heavy duty. At the 3/4-in row the diameter is 19.05 mm (unrounded 19.0500 mm), below 20.00 mm — so the 7/8-in row is the smallest table row whose diameter reaches 20.00 mm.

Largest table row whose diameter STAYS at or below 25.00 mm: At the displayed 7/8-in row the figures are 7/8 in = 22.23 mm (unrounded 22.2250 mm); heavy duty. At the 1-in row the diameter is 25.40 mm (unrounded 25.4000 mm), above 25.00 mm — so the 7/8-in row is the largest table row whose diameter stays at or below 25.00 mm.

Smallest table row that needs a heavy rope: At the displayed 7/8-in row the figures are 7/8 in = 22.23 mm (unrounded 22.2250 mm); heavy duty. At the 3/4-in row the diameter is 19.05 mm (unrounded 19.0500 mm), a medium rope — so the 7/8-in row is the smallest table row that needs a heavy rope.

A warning about the neighbours: every neighbour above sits one table row from the star, and every neighbour millimetre figure is a hair below or above the target even though it displays as the target — a one-row move changes the millimetres by more than a millimetre, but the unrounded figure still sits a hair from the round displayed figure, so the displayed two-decimal figure cannot tell the star from its neighbour. The page prints the unrounded four-decimal value in full so the rounding does not mislead you: 12.7000, 9.5250, 22.2250, 19.0500, 25.4000, 7.9375, 15.8750, 28.5750, 6.3500, 31.7500, 34.9250, 38.1000.

What the defaults assume

The defaults are a rope 1/2 inch in diameter — 1/2 in = 12.70 mm; medium duty. With 1/4 in, 1/4 in = 6.35 mm; light duty. With 5/16 in, 5/16 in = 7.94 mm; light duty. With 3/8 in, 3/8 in = 9.53 mm; light duty. With 1/2 in, 1/2 in = 12.70 mm; medium duty. With 5/8 in, 5/8 in = 15.88 mm; medium duty. With 3/4 in, 3/4 in = 19.05 mm; medium duty. With Diameter rows — 1/4 in, 1/2 in, 1-1/2 in, 1/4 in: 6.35 mm — light duty; 1/2 in: 12.70 mm — medium duty; 1-1/2 in: 38.10 mm — heavy duty. With Diameter 0 in, 0 in = 0.00 mm; matches no standard class — no diameter at all, and no matching class. With Diameter -0.5 in, 0 in = 0.00 mm; matches no standard class — a negative diameter is floored to 0.00 mm, never a negative number. With Diameter 1/2 in, 1/2 in = 12.70 mm (unrounded 12.7000 mm); medium duty — the millimetre figure displays as 12.70 mm. With Diameter 3/8 in, 3/8 in = 9.53 mm (unrounded 9.5250 mm); light duty — the millimetre figure displays as 9.53 mm. With Diameter 1-1/2 in, 1-1/2 in = 38.10 mm (unrounded 38.1000 mm); heavy duty — the top of the table. With Diameter 0.45 in, 0.45 in = 12.70 mm; matches no standard class — in the gap between the light and medium classes, so no standard class. With Diameter 0.8 in, 0.8 in = 22.23 mm; matches no standard class — in the gap between the medium and heavy classes, so no standard class. With Diameters 1/4, 3/8, 1/2, 3/4, 7/8, 1-1/2, 2 and 0.2 in, 1/4 in = 6.35 mm — light duty; 3/8 in = 9.53 mm — light duty; 1/2 in = 12.70 mm — medium duty; 3/4 in = 19.05 mm — medium duty; 7/8 in = 22.23 mm — heavy duty; 1-1/2 in = 38.10 mm — heavy duty; 2 in = 38.10 mm — matches no standard class; 0.2 in = 6.35 mm — matches no standard class — the bottom of the light class, the top of the light class, the bottom of the medium class, the top of the medium class, the bottom of the heavy class, the top of the heavy class, past the heavy class, and below the light class. With Diameter 5/16 in, 5/16 in = 7.94 mm (unrounded 7.9375 mm); light duty — the millimetre figure displays as 7.94 mm. With Diameter 5/8 in, 5/8 in = 15.88 mm (unrounded 15.8750 mm); medium duty — the millimetre figure displays as 15.88 mm. With Diameter 1-1/8 in, 1-1/8 in = 28.58 mm (unrounded 28.5750 mm); heavy duty — the millimetre figure displays as 28.58 mm.

A note on the rounding: the millimetre figure is displayed to two decimals, half-up. With a 1/2-in rope the unrounded figure is 12.7000 mm — it displays as 12.70 mm. With a 3/8-in rope the unrounded figure is 9.5250 mm — it displays as 9.53 mm. With a 5/16-in rope the unrounded figure is 7.9375 mm — it displays as 7.94 mm. With a 5/8-in rope the unrounded figure is 15.8750 mm — it displays as 15.88 mm. With a 1-1/8-in rope the unrounded figure is 28.5750 mm — it displays as 28.58 mm. The page prints the unrounded value so the rounding does not mislead you.

Output · formula · meaning

Who uses this

• The cruiser buying a new dock line: they learn a 1/2-inch rope is 12.70 mm — medium duty — and they check the 1/2-in row to see the smallest row whose diameter reaches 10.00 mm.
• The daysailer sizing a light-duty line: they learn the light class is 1/4–3/8 in and below 3/8 in the rope is a light-duty line that parts under real load — and they check the 1/2-in row to see the smallest row that needs a medium rope.
• The liveaboard fitting heavier deck gear: they learn above 7/8 in the rope needs a windlass and heavier deck gear — and they check the 7/8-in row to see the smallest row whose diameter reaches 20.00 mm.
• The student converting inches to millimetres: they learn the millimetres are the inches times 25.4 — and they check the 7/8-in row to see the smallest row that needs a heavy rope.

Accuracy & limitations

The math is first-principles: the millimetres are the diameter in inches times 25.4, and the class comes from the closed bands — light 1/4–3/8 in, medium 1/2–3/4 in, heavy 7/8–1-1/2 in (rope DIAMETER) — all in inches and millimetres, the millimetre figure displayed to two decimals half-up. There is no division in this calculator — nothing is ever undefined. A negative diameter is floored to 0.00 mm, never a negative number. With a 1/2-in rope the unrounded figure is 12.7000 mm — it displays as 12.70 mm. With a 3/8-in rope the unrounded figure is 9.5250 mm — it displays as 9.53 mm. With a 5/16-in rope the unrounded figure is 7.9375 mm — it displays as 7.94 mm. With a 5/8-in rope the unrounded figure is 15.8750 mm — it displays as 15.88 mm. With a 1-1/8-in rope the unrounded figure is 28.5750 mm — it displays as 28.58 mm. 1/2 in is the standard rope for a cruising boat — the general-purpose dock-line and anchor-rode size, strong enough to hold in a blow without being too stiff to handle the way a 1-in rope is; below 3/8 in the rope is a light-duty line for small boats that parts under real load; above 7/8 in the rope needs a windlass and heavier deck gear. 1/4 in is the light floor — below it a cord does the job; 1-1/2 in is the practical ceiling — above it the rope is a ship's job, sized by classification rules, not a chart. This is why the medium zone runs 1/2–3/4 in while the light zone stops at 3/8 in — the medium rope is the cruising boat's rope, and 1/2 in is its middle. What the model cannot do is pick the rope for you: it says nothing about the rope material, the construction or the load — a 1/2-in rope that holds in a sheltered marina can part on an exposed mooring, and nylon stretches and absorbs shock while polyester holds its length and polypropylene floats but weakens in sun. Size up one row for exposed moorings or storm conditions, and always check the rope maker's own table for the exact product. Use it to check the size before you buy — then check the chain-size, screw-length and bolt-size charts before you set the rest of the ground tackle.

No rope-size page of any kind exists on the site — this is the first. The closest page is the chain size chart (CHAIN — the metal links that hold an anchor, not rope). This one is the ROPE diameter's own 12-row diameter → millimetres reference (1/4–1-1/2 in, inches and millimetres) with its three duty classes (light 1/4–3/8 in, medium 1/2–3/4 in, heavy 7/8–1-1/2 in) and class/gap logic.

Frequently asked questions