Farm Pond Calculator & Water Storage Capacity
Sizes farm ponds
“One accepted answer here saves a hundred people an hour each.
Size a farm pond or reservoir — from the dimensions get the storage capacityin m³, litres and gallons, the evaporation loss, and the irrigations and days of supply it provides.
Enter your pond
1 mm of water over 1 ha = 10 m³. Used to estimate irrigations supported and days of supply.
Next: line the pond (LDPE/HDPE or clay) to stop seepage, and where you can, reduce the open surface or add a floating cover — most loss is surface evaporation, so a deeper, narrower pond stores the same water with less daily loss.
Capacity uses the frustum (truncated-pyramid) formula V = h/3 · (A_top + A_bottom + √(A_top·A_bottom)). 1 mm over 1 ha = 10 m³; 1 m³ = 1000 L = 264.17 US gal.
Farm pond — key facts
- Volume
- frustum (truncated pyramid)
- Side slope
- 1:1 to 1:2 (H:V)
- 1 m³
- = 1000 L = 264 US gal
- 1 mm over 1 ha
- = 10 m³
- Evaporation
- ≈ 4–8 mm/day
- Acre-foot
- ≈ 1233 m³
- Cut losses
- deeper, narrower, lined
- Privacy
- Runs in your browser; nothing uploaded
Store the rain, water the dry spell
A farm pond is insurance against dry spells — it captures monsoon runoff and holds it for the weeks when the crop needs it most. To plan one you need its real storage capacity, and because a dug pond narrows towards the bottom, the right formula is the truncated-pyramid (frustum) volume, not a simple box. Get that wrong and you over- or under-build, or over-promise the area it can irrigate.
This tool computes the capacity in m³, litres and gallons from your top dimensions, depth and side slope, then estimates the surface evaporation loss and how many irrigations and days of supply it gives your crop. Use it to size a new pond, check an existing one, or decide whether to go deeper and narrower to cut evaporation. Line porous soils to stop seepage, and pair this with the Rainwater Harvesting and Irrigation Water tools to plan inflow and demand.
Size it correctly
Frustum maths gives the true capacity, not an over-stated box volume.
Plan the supply
See how many irrigations and days of water the pond provides.
Count the losses
Estimate evaporation so your supply plan is realistic.
Design smarter
Compare deep-narrow vs wide-shallow to minimise evaporation.
Frequently Asked Questions
How do I calculate a farm pond's capacity?+
A dug pond is shaped like an inverted truncated pyramid (wider at the top), so volume = depth/3 × (top area + bottom area + √(top × bottom area)). The bottom is smaller than the top because of the side slope. This tool does the full calculation from your top length, width, depth and slope and gives m³, litres and gallons.
What side slope should a farm pond have?+
Side slopes are usually 1:1 to 1:2 (horizontal:vertical) for stability — gentler in sandy or loose soils, steeper in firm clay. A 1:1.5 slope means the wall steps back 1.5 m for every 1 m of depth. Steeper slopes store more water per area but are more prone to collapse and harder to line.
How much water does a farm pond hold?+
It depends on size: a 30 × 20 m pond, 3 m deep with 1:1.5 sides holds roughly 1,200 m³ (about 1.2 million litres). The tool computes the exact figure for your dimensions and converts it to litres and US gallons.
How much water evaporates from a pond?+
Open water loses roughly 4–8 mm per day in warm, dry weather — so a pond with a 600 m² surface can lose around 3 m³ a day. Over a month that can be 5–15% of a shallow pond's volume. The tool estimates the daily loss and the monthly percentage from your surface area and evaporation rate.
How many irrigations will my pond support?+
Divide the pond volume by the water for one irrigation. One irrigation of D mm over A hectares needs A × 10 × D cubic metres (since 1 mm over 1 ha = 10 m³). The tool computes the per-irrigation volume from your area and application depth and shows how many full irrigations the pond holds.
How many days of water supply is that?+
The tool divides the pond volume by the daily demand (crop use over your area, plus evaporation). For example, a 1,200 m³ pond serving 1 ha at 5 mm/day (50 m³) plus ~3 m³ evaporation lasts about 22 days. It's a planning guide — actual life depends on rainfall, seepage and how you ration it.
Should I line my farm pond?+
If the soil is sandy or porous, yes — an HDPE/LDPE liner or compacted-clay/bentonite lining stops seepage that can otherwise lose more water than evaporation. In heavy clay soils a well-compacted pond may hold water without a liner. Lining also keeps stored water cleaner.
How can I cut evaporation losses?+
Make the pond deeper and narrower (less surface area per volume), provide windbreaks, and in small ponds consider floating covers or shade. Deep storage loses a smaller share of its volume than a shallow, wide pond of the same capacity — the tool's surface-area figure shows why.
Can I use this for a tank or reservoir?+
Yes — for a tank with near-vertical walls, set the side slope close to 0 and it becomes a simple box (volume = length × width × depth). For a dugout reservoir, use the real side slope. The frustum formula covers both cases.
How do I convert pond volume to gallons or acre-feet?+
1 m³ = 1000 litres = 264.2 US gallons. One acre-foot ≈ 1,233 m³. The tool reports m³, litres and US gallons; divide m³ by 1,233 for acre-feet if you plan in those units.
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