Wind pressure on sail calculator
20 knots on 30 m² of sail, square to the wind, push with about 1945 N, a little under 200 kgf. Add the height of the centre of effort and you also get the heeling moment before deciding to reef. Free in your browser.
- Free
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- Your data stays on your device
Run — free
Worked example
- Wind speed kn
- 20
- Sail area m2
- 30
- Wind speed ms
- 10.28888889
- Air density kg m3
- 1.225
- Pressure coefficient
- 1
- Wind angle deg
- 90
Sailing skippers, yacht designers, and riggers need a first-order number for how hard the wind pushes on canvas before they size sheets, check heeling, or decide when to reef. This wind pressure on sail calculator converts true wind speed in knots to metres per second with the international knot, builds dynamic pressure from one-half air density times velocity squared, scales that pressure by an optional shape coefficient and the sine-squared of the wind-to-sail angle, and multiplies by sail area to give total force in newtons, kilograms-force, and pounds-force. Optional centre-of-effort height turns that force into a heeling moment for a simple righting-arm comparison.
How wind pressure on a sail is estimated
A sail is a thin lifting surface, but for rigging loads and a first heeling check the useful starting point is still the dynamic pressure of the oncoming air. Dynamic pressure q is one-half times air density times v squared. At sea-level International Standard Atmosphere conditions the default density is 1.225 kilograms per cubic metre, which is dry air at fifteen degrees Celsius; hotter or higher sailing wants a lower density because force falls with rho.
Effective wind pressure on the cloth is q times a dimensionless pressure coefficient Cp, defaulting to one for a perpendicular flat-plate estimate. Typical working sails sit near one to one-and-a-half once you fold in camber and a filled shape, which is why this field is exposed rather than hidden. The force normal to the sail then follows the classical inclined-plate law: F equals P times A times the square of the sine of the angle between the wind and the sail plane.
At ninety degrees — a beam reach with the sail standing across the flow — sine is one and you recover the full F equals P times A figure used in many dockside rules of thumb. At thirty or forty-five degrees the same area produces a much smaller sideways force, which is why a close-hauled boat heels less than a beam-reach boat in the same true-wind speed.
When you also know the height of the sail centre of effort above the underwater centre of lateral resistance, the heeling moment is simply force times that height, in newton-metres. This is an estimate for comparison and gear talk, not a substitute for a velocity-prediction program, a finite-element mast model, or the builder's stability booklet.
What you can do with it
Reefing and sheet-load checks
Convert a forecast in knots and the working sail area into newtons and kilograms-force before you decide to reef, ease, or change down a headsail.
First-order heeling moment
Supply centre-of-effort height with wind speed and area to get a heeling moment in newton-metres for a simple comparison with the boat's righting arm.
Rigging and hardware sizing
Give riggers and chandlers a deterministic sail force from knots and area so blocks, tracks, and control lines are quoted against a number, not a Beaufort story.
FAQ
How do you calculate wind pressure and force on a sail?
Convert knots to m/s with the international knot (1852/3600), compute dynamic pressure q = ½ρv², scale by Cp, then F = P × A × sin²(θ).
What units do I enter for wind speed and sail area?
The response also reports m/s, pascals, pounds per square foot, newtons, kilograms-force, and pounds-force.
What is the pressure coefficient Cp?
The default 1.0 is a perpendicular flat-plate estimate. Typical working sails sit near 1.0–1.5; raise it for a conservative rigging load.
How is heeling moment calculated?
Omit the height and that field is left out of the response.
Why does the same wind push less on a close-hauled sail than on a beam reach?
Because the force follows the square of the sine of the angle between the wind and the sail. At 90 degrees you get the full pressure times area; at 30 degrees the same sail feels much less sideways force, so the boat heels less.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/travel/wind-force-on-sail \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"wind_speed_kn":20,"sail_area_m2":30}'const res = await fetch("https://api.kit.forhosting.com/travel/wind-force-on-sail", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"wind_speed_kn": 20,
"sail_area_m2": 30
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/travel/wind-force-on-sail",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"wind_speed_kn": 20,
"sail_area_m2": 30
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/travel/wind-force-on-sail", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"wind_speed_kn":20,"sail_area_m2":30}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"wind_speed_kn":20,"sail_area_m2":30}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/travel/wind-force-on-sail", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"wind_speed_kn": 20,
"sail_area_m2": 30
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "travel.wind_force_on_sail",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
Invalid input is not billed; successful calls use the published $0.002 base price.
How wind pressure on a sail is estimated
Convert knots to metres per second with the international definition of the knot: one nautical mile of exactly 1852 metres in one hour, so v equals wind_speed_kn times 1852 divided by 3600.
Input contract, units, and response fields
Zero knots is valid and returns zero force; a zero or negative area is rejected. Centre-of-effort and heeling-moment keys are omitted entirely when you did not send a height — they are never returned as null. Example: twenty knots on thirty square metres at the defaults yields about 64.84 pascals of dynamic pressure and about 1945 newtons of force, a little under two hundred kilograms-force, which is the order of magnitude a cruising main-and-jib pair feels in a fresh breeze. POST a JSON body with two required finite numbers: wind_speed_kn, the true wind speed in knots from zero to two hundred inclusive, and sail_area_m2, the sail area in square metres, strictly greater than zero and at most twenty thousand. Optional air_density_kg_m3 defaults to 1.225 and must sit between 0.5 and 2. Optional pressure_coefficient (aliases cp, cd, coefficient) defaults to 1 and must sit between 0.01 and 3. Optional wind_angle_deg is the angle between the wind and the sail plane, from 0 (edge-on, no force) to 180, defaulting to 90 so a first call without an angle reports full beam pressure. Optional center_of_effort_m is the height in metres of the centre of effort above the centre of lateral resistance; when set, the response includes heeling_moment_nm. Each numeric field may arrive as a JSON number or as a numeric string; missing required fields, empty strings, non-finite values, and numbers outside the stated ranges fail with invalid_input naming the field and are not billed. On success the body echoes the cleaned inputs, reports wind_speed_ms, dynamic_pressure_pa, wind_pressure_pa, wind_pressure_psf, projected_area_m2, force_n, force_kgf, and force_lbf, plus a formula string and a one-line summary.
Where a sail wind-force API earns its keep
Cruising skippers and sailing-school instructors use a wind pressure on sail calculator before they talk about reefing, sheet loads, or how hard a gust will press the rail down, because a twenty-knot beam reach on thirty square metres is a very different number from the same canvas close-hauled. Yacht designers and amateur builders need a deterministic first-order force and, when centre of effort is known, a heeling moment they can compare with a simple righting-arm estimate without opening a velocity-prediction program. Riggers and hardware shops size blocks, tracks, and control lines from a load in newtons or kilograms-force rather than from a Beaufort anecdote; feeding the same area and a conservative Cp through this endpoint keeps those quotes auditable. Fleet apps and passage-planning notebooks that already store forecast knots and a sail inventory can call a tiny parse API instead of re-deriving one-half rho v squared in every language binding. Keep requests small, pin force_n and wind_pressure_pa in tests, and you get reproducible rigging and heeling numbers for classrooms, LMS plugins, and edge workers without standing up a naval-architecture stack of your own. This endpoint does not predict boat speed, leeway, or capsize: it only converts the wind and canvas you already chose into pressure and force.
Technical questions
What does the wind pressure on sail calculator cost?
The browser tool is free. API requests use prepaid KIT balance at $0.002 per successful call; invalid input is not billed.