TryBuildCalcCircular Pond Liner Calculator (Diameter & Square Liner Sizing)
Size a round pond's volume and liner instantly.
🕒 Last updated: September 23, 2026
Inputs
ℹ️Rectangular and Circular use exact geometric volume; Oval uses the exact ellipse-area formula; Irregular/Kidney uses a disclosed mid-range approximation, since no exact formula exists for a freeform shape.
ℹ️The true AVERAGE depth across the whole basin, not the deepest point — sloped walls mean average depth is genuinely shallower than max depth. Liner sizing separately uses your pond's maximum depth below.
ℹ️The depth at your pond's single DEEPEST point — used only for liner sizing, since the liner has to physically reach the bottom and back up. Must be at least the average depth above (equal for straight, vertical-walled ponds).
Marginal Shelf Zone
A shallow planting ledge around the perimeter, at its own shallower depth — its footprint is subtracted from the main basin, not added on top.
Pond Liner
ℹ️Extra material left at the edge for anchoring/trimming — 2 ft is a commonly-cited standard, adjustable here.
Pump / Filtration
ℹ️Drives both the pump turnover-rate target and which fish-stocking guideline (volume-based or surface-area-based) is shown.
Cost
Water Volume
1,469
US gal (5,560 L, 196.3 cu ft)
Liner Size Needed
18 x 18
ft (346.7 sq ft to purchase)
Recommended Pump
2,203
GPH rated (min. 1,469 GPH delivered)
Shape: Circular — Pond Type: Goldfish Pond — Lightly Stocked
Pond Geometry
Diameter: 10 ft
Average depth: 2.5 ft / Maximum depth: 3 ft
Surface area: 78.5 sq ft (7.3 m²)
Water Volume
1,469 US gal (1,223 Imperial gal)
5,560 litres (5.56 m³)
Cubic feet: 196.3 cu ft (7.27 cu yd)
Liner & Underlayment
Bounding size: 10 x 10 ft, max depth 3 ft, overlap 2 ft
Liner cut size: 18 x 18 ft (324 sq ft)
Purchase area (incl. 7% waste): 346.7 sq ft
Underlayment: 346.7 sq ft (same footprint as liner)
Pump / Filtration
Turnover target: 1x/hr (Goldfish Pond — Lightly Stocked)
Minimum delivered flow: 1,469 GPH
Recommended rated pump: 2,203 GPH
Fish Stocking Guideline
Roughly 146.9 in of fish (volume-based). Roughly 1 inch of fish per 10 gallons of water volume (general goldfish/mixed-pond planning guideline).
⚠ General planning guideline only — filtration, aeration, and fish's eventual mature size also affect real safe stocking.
Assumptions Used
Circular pond, average water depth (not maximum depth). Volume uses an exact geometric formula for this shape. No marginal shelf is included. Liner sized using bounding length/width, the pond's maximum depth (not average), and a 2 ft overlap allowance, plus a 7% purchase waste allowance; underlayment reuses the same purchase area. Pump sized from a 1x/hr turnover target for Goldfish Pond — Lightly Stocked, with a recommended rated spec at 1.5x the minimum delivered flow to account for head-height/pipe-friction losses. This calculator computes volume, liner, and pump-sizing estimates only — it does not verify excavation, structural, electrical, or local permitting requirements.
Approximate results for planning only. Verify with a professional.
Diagram simplified for clarity (not to scale) — depth bands reflect relative depth, not exact scale.
Looking for the verification checklist, reference tables, tips, or common mistakes?See the complete Pond/Water-Feature Volume Calculator.
Why a circular pond's liner is cut as a square
A circular pond's volume is an exact cylinder calculation — pi x radius squared x average depth — but the liner itself is cut as a SQUARE sized to the pond's own diameter, not a circle, since a flat liner sheet naturally drapes into a round excavation from a square starting cut, per standard pond-liner supplier guidance.
This page defaults to a 10 ft diameter, 2.5 ft deep pond — edit the inputs above to match your own round pond's size and depth.
Pond Calculator Formula: How Is It Determined?
Your pond's shape, dimensions, and average depth combine into a volume figure, which then drives liner sizing, pump GPH, and a fish-stocking guideline.
Step 1 — Volume by Shape
Rectangular/Square: Volume = Length x Width x Average Depth
Circular: Volume = pi x (Diameter/2)^2 x Average Depth
Oval/Elliptical: Volume = (pi/4) x Length x Width x Average Depth
Irregular/Kidney: Volume = Length x Width x Average Depth x 0.75 (disclosed mid-range estimate)
Rectangular, Circular, and Oval use exact geometric formulas; Irregular/Kidney uses a disclosed approximation since a freeform outline has no exact formula — break a highly irregular pond into rectangular sub-sections and sum them for a more accurate figure.
Step 2 — Marginal Shelf Zone (optional)
Main Basin Footprint = Footprint of the same shape, shrunk by (2 x Shelf Width)
e.g. Rectangular: (Length − 2 x Shelf Width) x (Width − 2 x Shelf Width)
e.g. Circular: pi x ((Diameter/2) − Shelf Width)^2
Shelf Footprint = Total Footprint − Main Basin Footprint
Total Volume = (Main Basin Footprint x Main Depth) + (Shelf Footprint x Shelf Depth)
The shelf's footprint is subtracted from the main basin — it occupies space that would otherwise be full-depth basin, not additional pond area. The shelf band is measured as the pond's own footprint minus the shrunk inner basin, rather than simply perimeter x shelf width, because that shortcut double-counts the corners where the band meets itself (a 12 x 8 ft pond with a 1 ft shelf is 36 sq ft of shelf, not the 40 sq ft the perimeter shortcut gives).
Step 3 — Liner Size
Liner Length = Bounding Length + (2 x Maximum Depth) + Overlap Allowance
Liner Width = Bounding Width + (2 x Maximum Depth) + Overlap Allowance
Uses the pond's MAXIMUM depth (deepest point, main basin or shelf), not average depth — the liner has to physically reach down and back up. A small purchase waste allowance is added on top; underlayment reuses the same purchase area.
Step 4 — Pump GPH
Minimum GPH = Volume (gal) x Turnover Rate (0.5x-2.0x/hr by pond type)
Recommended Rated GPH = Minimum GPH x 1.5
Waterfall (optional): + Spillway Width (in) x GPH per inch (100 light / 200 heavy)
Turnover rate scales with stocking level; the 1.5x factor accounts for head-height/pipe-friction losses between a pump's printed rating and its real delivered flow. A waterfall adds its own GPH on top, reported as a separate line item.
Step 5 — Fish Stocking Guideline
Goldfish/general (volume-based): Max Fish Inches = Volume (gal) / 10
Koi (surface-based): Max Fish Inches = Surface Area (sq ft) / 1
A general planning guideline only — only the rule matching your selected pond type is shown.
Step 6 — Cost
Line Cost = Purchase Quantity (sq ft/estimate) x Unit Price
Total = Liner + Underlayment + Pump
Every cost line is priced off the actual purchase quantity, and every cost field is zero unless cost estimation is enabled.
Worked Example
This example walks through your current inputs above, using the same steps as the Formula section.
Input Values Used
| Input | Value |
|---|---|
| Pond Size | 10 ft diameter, avg depth 2.5 ft, max depth 3 ft |
| Shape | Circular |
| Pond Type | Goldfish Pond — Lightly Stocked |
Step 1 — Volume
| Calculation | Result |
|---|---|
| 78.5 sq ft surface x depth | 196.3 cu ft (1469 US gal) |
Step 3 — Liner Size
| Calculation | Result |
|---|---|
| 10 + (2 x 3) + 2 ft overlap | 18 x 18 ft (346.7 sq ft to buy) |
Step 4 — Pump GPH
| Calculation | Result |
|---|---|
| 1469 gal x 1x/hr | 1469 GPH min (2203 GPH recommended) |
Figures above are rounded for display; the real quantities are always calculated from full, unrounded values.
Therefore, a 78.5 sq ft circular pond at 2.5 ft average depth holds 1469 US gal (5560 L), needs a 18 x 18 ft liner, and a pump rated at least 2,203 GPH.