How it works
Find the area of the field in square feet with the formula for its shape, then divide by 43,560, the number of square feet in an acre. Enter every dimension in the same unit; the metric toggle works in meters and gives hectares.
Irregular fields can be split into rectangles and triangles: calculate each and add the results.
Acres = area (ft²) ÷ 43,560
Rectangle: L × W
Triangle: ½ × base × height
Circle: π ÷ 4 × D²
Trapezoid: (a + b) ÷ 2 × h
Worked example
A field 660 ft long and 66 ft wide (one chain by one furlong):
- Area = 660 × 66 = 43,560 ft²
- Acres = 43,560 ÷ 43,560 = 1 ac
| Input | Value |
|---|---|
| Field shape | Rectangle (length × width) |
| Length, base, diameter, or first parallel side | 660 ft |
| Width, height, or second parallel side | 66 ft |
| Height between the parallel sides (trapezoid only) | 100 ft |
| Result | Value |
|---|---|
| Area | 1 ac |
| Area | 43,560 ft² |
| Side of a square with the same area | 208.7 ft |
Assumptions and limits
- Flat, level ground measured horizontally. Slope lengths overstate the map area.
- Property lines and legal descriptions govern acreage for sales, taxes, and program payments; this tool is for planning.
- Straight boundaries; curves and irregular shapes must be split into pieces.
Common questions
How many square feet are in an acre?
43,560 square feet. A square acre is about 208.7 ft on a side.
How do I find acres from length and width?
Multiply the two in feet and divide by 43,560.
How do I measure an irregular field?
Break it into rectangles and triangles, calculate each, and add the areas.
What is a hectare?
A hectare is 10,000 square meters, or about 2.471 acres. Switch to Metric for hectares.
Sources
- NIST Handbook 44 and the definition of the acre: 43,560 square feet (an acre is 10 square chains, 660 ft by 66 ft).
- Area formulas for plane figures.
Updated 2026-09-30