Triangle area calculator
Get the area of any triangle from base and height, or from all three sides with Heron's formula — converted to square feet, square meters, and acres as you type.
Measure everything in the same unit; height must be perpendicular to the base. Nothing you type here leaves your browser.
Which formula to use
It comes down to what you can actually measure. If you can drop a perpendicular from the apex to the base — a garden bed against a wall, a gable end, anything with a right angle handy — the classic formula is all you need:
A 40 ft base with a 30 ft height is ½ × 40 × 30 = 600 ft². The catch is the word perpendicular. Measure the slanted edge instead of the true height and the answer is wrong every time.
Often you can't measure a height at all. A triangular lot, a corner of a field, a paddock — there's no apex to stand under, but you can walk the three edges with a tape or a measuring wheel. That's the surveyor's case, and it's what Heron's formula is for:
Sides of 30, 40, and 50 give s = 60, and √(60 × 30 × 20 × 10) = 600 ft² — the same triangle, because 30/40/50 is a right triangle with its height built in.
Turning area into acreage
One acre is 43,560 square feet. Odd triangular parcels are exactly where this matters: the calculator converts your area to square feet first (1 m = 3.28084 ft, 1 yd = 3 ft), then divides. That 600 ft² triangle is a rounding error of an acre — 0.0138 — but a 300 × 400 × 500 ft lot is 60,000 ft², or about 1.38 acres.
A note on precision
Tape error compounds. Being 2% short on each of three sides can shift a Heron's result by several percent, and on land that's real money. Pull the tape taut, keep it level, and measure each edge twice. If two readings disagree, believe neither — measure again.
Why this is on a forms site
Core Forms ships a data variables engine — the same live math runs inside WordPress forms. Quote calculators, area-based pricing forms, instant estimates for landscaping or flooring: this page is that idea standalone, and building your own takes a few fields and a computed variable, no JavaScript.
Triangle area questions
How do I calculate the area of a triangle?
If you can measure a perpendicular height, use area = ½ × base × height. A 40 ft base with a 30 ft height gives ½ × 40 × 30 = 600 square feet. If you only know the three side lengths, switch to Heron’s formula — the second mode above handles it.
What is Heron’s formula?
Heron’s formula finds a triangle’s area from its three sides alone. First compute the semi-perimeter s = (a + b + c) ÷ 2, then area = √(s(s−a)(s−b)(s−c)). For sides 30, 40, and 50, s = 60 and the area works out to 600 — no height measurement needed.
How do I convert square feet to acres?
Divide the square footage by 43,560 — the number of square feet in one acre. A 600 ft² triangle is 600 ÷ 43,560 ≈ 0.0138 acres. The calculator does this conversion automatically, converting meters or yards to feet first so the acre figure is always correct.
Can I use this for land or acreage?
Yes — that’s the main reason the Heron’s mode exists. Odd triangular lots rarely offer a clean perpendicular height, but you can always walk the three boundary lines with a tape or wheel. Enter the three sides in feet, meters, or yards and read the acreage tile directly.
What if I only know two sides and the angle between them?
This tool doesn’t have a trig mode, but the formula is simple enough to run by hand: area = ½ × a × b × sin(C), where C is the angle between sides a and b. Two 50 ft sides meeting at 60° give ½ × 50 × 50 × sin 60° ≈ 1,083 ft².