Land Area Calculator (4 Sides) – Result in Decimal & Katha

How to use

Choose the shape: rectangle, triangle, trapezoid, four unequal sides or round.

Choose the unit you measured in: feet, haat (cubit), yards, metres or chain.

Enter each side, matching the letters (a, b, c…) in the drawing.

For four unequal sides, also enter one diagonal — that makes the result exact.

See the area in decimal, katha, bigha, acre and square feet instantly.

Land area formulas

Shape	Formula
Rectangle / square	length × width
Triangle	√[s(s−a)(s−b)(s−c)], s = (a + b + c) ÷ 2
Trapezoid	(a + b) ÷ 2 × c
Four unequal sides	split into two triangles with a diagonal, then add
Round	3.1416 × radius × radius

From square feet to Bangladeshi units: decimal = sq ft ÷ 435.6, katha = sq ft ÷ 720, bigha = sq ft ÷ 14,400.

Examples

A rectangle measured in haat: 30 haat long, 20 haat wide. 30 haat = 45 ft, 20 haat = 30 ft. 45 × 30 = 1,350 sq ft. 1,350 ÷ 435.6 ≈ 3.1 decimal, and 1,350 ÷ 720 = 1.875 katha.

An irregular four-sided plot: a = 100, b = 60, c = 80, d = 50 ft, diagonal e = 110 ft. The two triangles add up to ≈ 4,809.5 sq ft ≈ 11.04 decimal. The average method gives 90 × 55 = 4,950 sq ft ≈ 11.36 decimal — about 2.9% too much.

What to know in Bangladesh

Length units: 1 haat = 18 inches = 1.5 ft; 1 yard = 3 ft; 1 metre ≈ 3.28 ft; 1 survey chain = 66 ft.
Land units: 1 decimal (shotok) = 435.6 sq ft, 1 katha = 720 sq ft, 1 bigha = 20 katha — the standard values. Local katha and bigha can differ by region.
Measure a diagonal: surveyors also split irregular plots into triangles. Keep the tape taut and level.
Legal area: the deed, khatian and mouza map are final; this calculator is for estimates.

More: why four sides alone can't give the area

Think of a door frame. Keep the four pieces the same, push one corner, and the frame leans — the space inside shrinks, yet no side changed length. Land works the same way: knowing four sides without the corner angles doesn’t fix the area.

One diagonal splits the plot into two triangles, and a triangle with three known sides can’t change shape. Heron’s formula then gives each triangle’s area exactly, and their sum is the plot’s area.

The common “average of opposite sides” shortcut is quick, but unless the corners are right angles it always overstates the area a little. For buying or selling land, measure a diagonal.

Frequently asked questions

What is the formula for land area?

Rectangle: length × width. Triangle: Heron's formula √[s(s−a)(s−b)(s−c)], where s is half the sum of the three sides. Trapezoid: (sum of the two parallel sides ÷ 2) × the distance between them. Divide square feet by 435.6 to get decimal.

How do I measure land when all four sides are different?

Besides the four sides, measure one diagonal from a corner to the opposite corner. It splits the plot into two triangles; adding their areas gives the exact area. Four sides alone are not enough, because the same four sides can enclose different areas depending on the corner angles.

Is the village 'average of sides' method accurate?

Many people multiply the average of two opposite sides by the average of the other two. It is close when the corners are nearly square, but it overstates the area of a slanted plot. This calculator shows both results and the difference.

How do I find the area of a triangular plot?

Measure all three sides, say 50, 60 and 70 ft. s = (50 + 60 + 70) ÷ 2 = 90. Area = √(90 × 40 × 30 × 20) ≈ 1,469.7 sq ft ≈ 3.37 decimal.

How do I calculate land measured in haat?

1 haat = 1.5 ft (18 inches). Multiply by 1.5 to get feet, or just choose 'Haat' as the unit in the calculator.

How is land measured with a chain calculated?

A Gunter's survey chain is 66 ft (100 links). 10 chains × 1 chain = 43,560 sq ft = 1 acre. Choose 'Chain' as the unit to enter chain measurements directly.

What if my result doesn't match the surveyor (amin)?

This calculator gives an estimate from the lengths you enter. The deed, khatian and official mouza map are legally final. For a big difference, have a licensed surveyor re-measure.