Sail Area Calculator
An 18 ft foot and a 22 ft height make a triangular sail of 198.00 sq ft, and most sailors have done that one division on a cockpit cushion at least once. This sail area calculator runs the flat geometry for three sail shapes, triangular, rectangular and trapezoidal, from the sail’s own edge lengths in feet. It returns square feet and square metres together, so a single run serves both a US sail loft quote and a metric rating form.
Square metres use the tool’s fixed conversion, square feet x 0.092903. The figure is the flat geometric area of the shape you picked: roach, camber and hollow are not in it. See the limits section below before quoting it as a rated area.
Quick Answer
A triangular sail with an 18 ft base and a 22 ft height has an area of 198.00 sq ft (18.39 sq m): 18 × 22 ÷ 2. The same calculator handles rectangular sails (length × width) and four-sided trapezoidal sails ((top + bottom) × height ÷ 2), and it always prints square feet and square metres side by side. Those are the pre-filled values above, so one click on Calculate reproduces them.
Key takeaways
- Three flat shapes cover most real sails: triangular for mainsails and jibs, trapezoidal for gaff sails, rectangular for square sails and flat panels.
- Every length goes in as feet. There is no unit switch, and the square metre readout is a fixed conversion (sq ft × 0.092903), not a second measurement.
- Flat geometry ignores roach. A rated mainsail area from a measurer will usually come out larger than this calculator’s number for the same sail.
- Total working sail area is a sum, not a setting: run the mainsail and the foretriangle separately and add the two results.
- Quick Answer
- How to use the sail area calculator
- The three sail area formulas this tool runs
- Worked example: an 18 by 22 ft triangular sail
- Measuring a real sail: roach, LP and why rated areas run bigger
- Total sail area, and what the number is good for
- What competing calculators miss
- The honest limits of a flat-shape sail calculator
- Frequently asked questions
- Add This Calculator to Your Website
How to use the sail area calculator
Pick the shape, type the edge lengths in feet, and click Calculate. The results stay hidden until a run succeeds.
- Choose the sail type: Triangular, Rectangular or Trapezoidal. The input panel changes to match, showing two fields for a triangle or rectangle and three for a trapezoid.
- Enter the lengths in feet: base and height for a triangle, length and width for a rectangle, top base, bottom base and height for a trapezoid. Decimals work, so 18.5 and 22.75 are fine.
- Click Calculate sail area: the two readout fields fill in together, square feet first and square metres second, both rounded to two decimals.
- Copy or share the pair with the buttons in the results box, which keeps the two units together for a sailmaker or a class measurer.
Anything that cannot produce a positive area is rejected with the alert “Please enter valid measurements”: a blank field, a text entry, a zero, or a single negative length all land there, because the tool reads a missing or unreadable entry as 0. One quirk is worth knowing up front, and the limits section below repeats it: two negative lengths multiply into a positive area, so the tool will happily report 198.00 sq ft for a base of -18 and a height of -22. That is a typing error to fix, not a measurement.
The three sail area formulas this tool runs
Each mode is one line of school geometry, applied to the sail’s own edges. The table below is the whole tool, with nothing hidden.
| Shape | Inputs the tool reads | Formula | Sails it fits |
|---|---|---|---|
| Triangular | Base, Height | (Base × Height) ÷ 2 | Bermuda mainsails, jibs and genoas treated as flat triangles |
| Rectangular | Length, Width | Length × Width | Square sails, lateen yards set flat, shade and cover panels |
| Trapezoidal | Top base, Bottom base, Height | ((Top + Bottom) × Height) ÷ 2 | Gaff sails and other four-sided sails with two parallel edges |
The vocabulary maps straight onto rig dimensions. On a mainsail the base is the foot (the E measurement) and the height is the luff up the mast (P), which is why the standard mainsail formula quoted by Cruising World’s guide to measuring sail area is P × E ÷ 2. For a headsail the same triangle is the foretriangle, I × J ÷ 2. Height always means the perpendicular distance, not the sloping leech: measure up the mast or across the floor, never along the curved back edge of the sail.
Worked example: an 18 by 22 ft triangular sail
The pre-filled example is a mid-size cruising mainsail treated as a flat triangle: an 18 ft foot and a 22 ft height. Running it gives 198.00 sq ft and 18.39 sq m.
Three more runs, one per shape plus a decimal case, check the rest of the tool. Every row below came out of this exact calculator, so you can retype any of them and land on the same figures.
| Run | Inputs (ft) | Area (sq ft) | Area (sq m) |
|---|---|---|---|
| Triangular, worked example | Base 18, Height 22 | 198.00 | 18.39 |
| Triangular, decimal edges | Base 10.5, Height 15.25 | 80.06 | 7.44 |
| Rectangular panel | Length 12, Width 10 | 120.00 | 11.15 |
| Trapezoidal gaff sail | Top 6, Bottom 14, Height 20 | 200.00 | 18.58 |
Measuring a real sail: roach, LP and why rated areas run bigger
A real sail is not a flat triangle, and the gap between the two numbers is systematic, not an error. Three separate effects push a measured or published area above the flat figure this tool returns.
Roach. The leech of a mainsail curves outward beyond the straight head-to-clew line, and that crescent of cloth is real area the triangle formula never sees. Cruising World’s measurement guide notes the conventional mainsail triangle excludes roach entirely, the old IOR rule held roach to roughly 11 percent of the triangle with limited battens, and full-batten sails carry far more. The Sail-World guide to calculating sail areas, working from ORC practice, prices a roached mainsail at P × E × 0.585 against the bare triangle’s P × E ÷ 2.
Headsail overlap. A genoa does not stop at the foretriangle. Headsails are sized by LP, the shortest distance from clew to luff, quoted as a percentage of J; a 150 percent genoa carries half as much cloth again across that perpendicular as the triangle suggests. The Mandurah Yacht Academy’s sail measurement codes guide sets out the LP, luff and girth definitions the rating rules actually use.
Measurement conditions. Formal measurement happens with the sail dry, off the spars, laid flat with just enough tension to pull the wrinkles out, per the Equipment Rules of Sailing. A sail measured slumped on a dock, or worse, estimated from a brochure photo, starts from different numbers before any formula runs. Measure your own sail the same way the rules do: flat, dry, edges pulled straight, and record tack-to-clew and tack-to-head as straight lines.
Total sail area, and what the number is good for
Total working sail area is the sum of the parts, conventionally the mainsail triangle plus the foretriangle. Run each in triangular mode and add them: a boat with P 30 ft and E 12 ft (180.00 sq ft of mainsail triangle) and I 36 ft and J 11 ft (198.00 sq ft of foretriangle) works out to 378.00 sq ft of nominal working area.
That total earns its keep in comparisons. Divided into the sail area/displacement ratio it sketches how powered-up a design is: the Rustler Yachts guide to boat-choice formulas puts offshore cruisers happiest around 15 to 18, boats at 12 and below underpowered, and 20 and above in racing territory. Treat the bands as a yardstick for comparing two boats measured the same way, never as a speed prediction: hull shape, displacement and crew do the rest. If you are weighing a used boat, run her sail plan through this page before you set the result against a boat value calculator estimate or a boat price calculator figure, because a tired, undersized sail inventory is a real cost hiding inside a fair-looking price.
Area is also only half the performance story. How much of the wind drives the boat depends on its angle to the course, the same component split a crosswind calculator works through, which is why two boats with identical sail area can feel completely different on a reach versus a beat.
What competing calculators miss
Sail area tools split into rig-design suites and one-formula pages, and each camp gives something up:
- Brave Calculator: frames the job by sail type (mainsail, jib, spinnaker) but its published triangular formula, A = L × P ÷ 2, is a triangle whatever the label says. Calling that a spinnaker calculation understates a balloon sail badly; spinnakers are sized from girths.
- Area Calculators: one formula only, SA = H × F ÷ 2, with metric worked examples (10 m by 5 m, 15 m by 8 m). No rectangular or trapezoidal mode, no dual-unit readout, and nothing about roach.
- Good Old Boat / Sailrite sail calculator: the deepest of the set, with a boat database, I/J/P/E rig dimensions, LP percentages and roach-aware areas. The trade-off is that it designs around a known boat’s rig; when a physical sail is lying on the floor and you need its own flat area, or the panel is a trapezoid, the rig suite is the wrong shape of tool.
- SailMentor: turns I, J, P and E into total working area plus a sail area/displacement ratio, which is genuinely useful for comparing boats. It never measures an individual sail from its own edges, and it has no rectangular or trapezoidal mode.
- Shade-sail calculators: Heron’s formula, perimeters and diagonals for tensioned garden shades. Correct math aimed at a different job, with no sail-specific guidance at all.
- The gap this page fills: three flat shapes in one small tool, square feet and square metres on every result, worked checks you can retype, and the roach and rating limits stated on the page instead of left for the reader to discover at the measurer’s table.
The honest limits of a flat-shape sail calculator
This tool measures flat shapes, and only flat shapes. Keep four limits in mind before you quote its numbers anywhere that matters.
- No roach, no camber, no hollow. The curved leech of a mainsail, the belly of a genoa and the hollow cut into a foot all sit outside a triangle, rectangle or trapezoid. A sailmaker cutting cloth or a measurer issuing a rating certificate works from width and girth measurements this tool does not take.
- No spinnakers. Symmetric and asymmetric spinnakers are sized by rule formulas built on luff length and mid-girth. Typing a spinnaker’s foot and luff into triangular mode produces a number, and it will be far too small.
- Feet in, both areas out. Every field is read as feet; there is no metres or inches mode. The square metre column is a conversion of the square foot result, so convert metric lengths to feet before typing them (multiply by 3.28084) or the conversion column will faithfully convert the wrong area.
- Signs are on you. A single negative length or a zero triggers the alert, but two negative lengths multiply into a positive area and return a result that looks perfectly sensible. Glance at the signs before you write the number down.
None of that makes the flat figure useless. It is the number sail plans are compared with, the starting point for cloth estimates (add seam, hem and reinforcement allowances on top, and round up), and the quickest sanity check on a broker’s advertised sail area: if a listing claims far more than the triangles add up to, the overlap and roach story had better be a good one.
Frequently asked questions
Multiply the base by the height and divide by 2. An 18 ft base and a 22 ft height give 18 x 22 / 2 = 198 sq ft. In this calculator, pick Triangular Sail, enter those two lengths in feet, and it returns 198.00 sq ft and 18.39 sq m.
Add the top base and the bottom base, multiply by the height, and divide by 2. A gaff sail with a 6 ft top, a 14 ft bottom and a 20 ft height gives (6 + 14) x 20 / 2 = 200 sq ft. Pick Trapezoidal Sail in the calculator and enter the three lengths in feet.
The inputs are in feet only, and there is no unit selector. It always shows two outputs, square feet and square metres, so if you measured in metres, convert each length to feet first (multiply by 3.28084) and then read the square metre result. Entering metre values as if they were feet gives a wrong area.
Real mainsails curve out beyond the straight line from head to clew. That extra cloth, the roach, sits outside the flat triangle this calculator measures, and full-batten sails carry more of it. Rating rules price the roach in with width measurements; a flat base x height / 2 figure is the nominal triangle area, not the rated area.
Run each sail separately and add the results. The long-standing convention for working sail area adds the mainsail triangle (luff x foot / 2) to the foretriangle (height x base / 2), which is the triangular mode used twice. Overlapping genoas and roach add real area beyond those two triangles, so treat the total as a comparison figure.
No. Spinnakers are balloon-shaped, and rating rules size them from luff lengths and girths, not from a triangle, rectangle or trapezoid. This tool covers flat triangular, rectangular and trapezoidal sails only. A spinnaker figure built from edge lengths alone would badly understate the cloth.
If the entries do not produce a positive area, the calculator shows the alert Please enter valid measurements and adds no new result. One honest quirk: two negative lengths multiply into a positive area, so a result computed from negative entries is a typing error, not a real measurement. Check the signs before you trust the number.
Yes. It runs in your browser, needs no account and stores nothing. Use Copy my results or Share results in the results box to keep the square foot and square metre figures together.
Add This Calculator to Your Website
Embed this calculator on your own site with the code below. Pick a size to preview it live, then copy the code.
Powered by Calculator Zilo
References
- Cruising World. Measuring Sail Area (the conventional working-area method, SA = I x J / 2 + P x E / 2, LP percentages for headsails, and roach excluded from the mainsail triangle). Accessed October 24, 2026.
- Sail-World. Calculating your sail areas: a guide by upffront.com (ORC-based sail area calculations, roached mainsail factor P x E x 0.585, and the measurement codes for headsails and spinnakers). Accessed October 24, 2026.
- Mandurah Yacht Academy. Understanding Sail Measurement Codes (definitions of LP, luff, foot and spinnaker girths, and how IRC, ORC and PHRF use them). Accessed October 24, 2026.
- Rustler Yachts. 3 useful formulas to help you choose a boat (the sail area/displacement ratio, its formula, and the cruising bands quoted in this article). Accessed October 24, 2026.
- Sailboat Cruising. Sail Area Calculations: A Simple Guide for Sailors (mainsail roach adjustments and genoa percentage calculations in plain language). Accessed October 24, 2026.