A flexible pond liner must cover the bottom, run up the sides at the deepest point and leave an overlap of about 1–2 ft around the edge to anchor under stones. A pump that circulates the whole volume roughly once every hour (every 2 hours for plant ponds, faster for koi) keeps water clear with a filter.
How to use the Pond Calculator
- Shape
- Length
- Width
- Average depth
- Maximum depth
- Edge overlap
- Liner price
- Water price
- Every result field is editable: type into Water volume and Average depth plus every other field update to match.
Results update as you type, and every field accepts the units shown in its menu.
How it works
Choose the pond shape, enter the length, width, average and maximum depth, the edge overlap and prices for liner and water.
Tips for an accurate estimate
- Measure inside dimensions of tanks and pools – wall thickness can change the volume noticeably.
- Use the average depth for pools with a sloping floor.
- Remember the weight: water weighs about 8.34 lb per US gallon (1 kg per litre), so large volumes need solid support.
- Check whether a figure is in US gallons, imperial gallons or litres before ordering chemicals or equipment.
Common mistakes to avoid
- Using wall height instead of water depth for above-ground pools.
- Assuming a horizontal tank’s volume is proportional to the liquid level – it is not.
- Mixing up flow units such as GPM, GPH and litres per minute.
Inputs explained
| Field | What to enter |
|---|---|
| Shape | Choose one: Rectangle, Oval / ellipse, Circle (length = diameter) |
| Length | Units: ft, m, in, yd, cm |
| Width | Units: ft, m, in, yd, cm |
| Average depth | Units: ft, m, in, yd, cm |
| Maximum depth | Units: ft, m, in, yd, cm |
| Edge overlap | Units: ft, m, in, yd, cm |
| Liner price | Units: /ft², /m², /yd² |
| Water price | Units: /1000 gal, /m³ |
What each result means
- Water volume = [Surface area]×[Average depth]
- Liner length = [Length] + 2×[Maximum depth] + 2×[Edge overlap]
- Liner area = [Liner length]×[Liner width]
- Liner cost = [Liner area]×[Liner price]
- Water cost = [Water volume]×[Water price]
- Pump size (1 turnover per hour) = [Water volume]÷3600
Example values
How the main result changes when Average depth changes and everything else stays at the example values above:
| Average depth (ft) | Water volume (US gal) |
|---|---|
| 1 | 449 |
| 1.5 | 673 |
| 2 | 898 |
| 2.5 | 1,122 |
| 3 | 1,346 |
| 4 | 1,795 |
Formula
Volume = surface area × average depthLiner length = L + 2 × max depth + 2 × overlap; liner width = W + 2 × max depth + 2 × overlapPump flow ≈ pond volume per hourWorked example
Inputs — Shape: Rectangle; Length: 10 ft; Width: 6 ft; Average depth: 2 ft; Maximum depth: 2.5 ft; Edge overlap: 1 ft; Liner price: $0.8 /ft²; Water price: $5 /1000 gal.
Results — Surface area = 60 ft²; Water volume = 898 US gal; Liner length = 17 ft; Liner width = 13 ft; Liner area = 221 ft²; Liner cost = $176.80; Water cost = $4.49; Pump size (1 turnover per hour) = 898 gph.
Frequently asked questions
How do I calculate pond liner size?
Length + 2 × deepest depth + 2 × overlap, by width + 2 × deepest depth + 2 × overlap.
How many gallons is my pond?
Length × width × average depth (ft) × 7.48 for a rectangle; multiply by 0.785 for an oval.
What size pump do I need?
One that moves the whole volume about once an hour at the height you need to lift it.
What formula does the Pond Calculator use?
Volume = surface area × average depth.