How it works
A pond is a solid whose volume is the surface area times the average depth. Approximate the shoreline with a rectangle, an ellipse, or a circle, measure the average depth by sampling, and multiply. Dividing cubic feet by 43,560 gives acre-feet, the unit used for water rights and chemical dosing.
Because banks slope and bottoms vary, the average depth is always less than the deepest reading. Errors in average depth dominate the answer, so take several measurements across the pond.
Worked example
An oval pond 200 ft by 120 ft with a 5 ft average depth:
- Surface area = π ÷ 4 × 200 × 120 = 18,850 ft² (0.43 ac)
- Volume = 18,850 ft² × 5 ft = 94,248 ft³ = 2.16 acre-ft
- Gallons = 94,248 × 7.48052 = 705,022 gal
| Input | Value |
|---|---|
| Pond shape | Oval or ellipse |
| Length, or diameter of a circle | 200 ft |
| Width (not used for a circle) | 120 ft |
| Average depth | 5 ft |
| Result | Value |
|---|---|
| Volume | 705,022 gal |
| Volume in acre-feet | 2.16 acre-ft |
| Surface area | 0.43 ac |
| Volume | 94,248 ft³ |
Assumptions and limits
- The shore is treated as one simple shape; irregular ponds should be split into pieces or measured more carefully.
- Average depth is your measurement, not calculated. It is the main source of error.
- Treatment rates for weeds, algae, or fish come from the product label and your extension office; none is built in.
Common questions
How many gallons is an acre-foot?
325,851 gallons, or 43,560 cubic feet.
How do I find my pond’s average depth?
Measure depth at many spots in a grid or transect and average them. The deepest spot alone overstates the volume.
How do I calculate acres from feet?
Divide the surface area in square feet by 43,560.
Can I use this to dose a pond treatment?
Use it to estimate volume only; the product label sets the dose and any restrictions.
Sources
- Penn State Extension: Pond Measurements (volume = surface area × average depth).
- 1 acre-foot = 43,560 ft³ = 325,851 US gal (43,560 × 7.48052).
- Read them: Penn State Extension: Pond Measurements: Area, Volume and Residence Time
Updated 2026-09-30