Beginner’s Guide: How to Calculate Pond Gallons & Pick the Perfect Pond Liner Size

Beginner’s Guide: Pond Gallons, Liner Size & Underlayment

Beginner’s Guide: Pond Gallons, Liner Size & Underlayment

New to ponds? You’ve got this. Use the diagrams and step-by-step formulas to measure, calculate gallons, pick a liner (EPDM or RPE), plan your waterfall, and choose underlayment—minus the math headaches.

Pro tip: Build in extra. Our liner sizing adds ~+1 ft on each side (already baked into the formula). Trim only after filling and settling.

Step 1 — Measure Your Pond (Pick the Closest Shape)

Measure maximum length & width, and use average depth if your bottom slopes.

Rectangle
Rectangle: measure Length (L), Width (W), Average Depth (D) Length (L) Width (W) Depth (D avg)

Measure: L (max), W (max), D (avg)

Circle
Circle: measure Diameter (Dia), Average Depth (D) Diameter Depth (D avg)

Measure: Diameter, D (avg)

Oval
Oval: measure Length (L), Width (W), Average Depth (D) Length (L) Width (W) Depth (D avg)

Measure: L (max), W (max), D (avg)

Kidney
Kidney: use max Length (L), max Width (W), Average Depth (D) Length (max) Width (max) Depth (D avg)

Approximate like an oval using your max L & W.

Peanut
Peanut: treat as an oval with max L & W; Depth (avg) Length (max) Width (max)

Use max L & W; Depth is average.

Lagoon
Lagoon: freeform—use max L & W; Depth (avg) Length (max) Width (max)

Use max L & W; Depth is average.

L-Shaped
L-Shape: treat as two rectangles—sum volumes Two rectangles → add

Break into two rectangles; measure each, then add.

Average depth tip: Take 3–4 readings around the pond and average them.

Step 2 — Calculate Pond Gallons

Use the formula for your shape. 1 cubic foot = 7.48 US gallons.

  • Rectangle: L × W × D × 7.48
  • Circle: (π × (Dia/2)2 × D) × 7.48
  • Oval / Kidney / Peanut / Lagoon (oval approx.): (π/4 × L × W × D) × 7.48
  • L-Shaped: ((L1×W1×D1) + (L2×W2×D2)) × 7.48
Example (Oval): L=12′, W=8′, D=2.5′ → Gallons ≈ (0.785 × 12 × 8 × 2.5) × 7.48 ≈ 1,760 gallons.

Step 3 — Calculate Liner Size (Built-In Extra + Rounds to 5-ft)

Use your pond’s max length & width and average depth. We add ~1 ft each side for safety.

  • Liner Length = Pond Length + (2 × Max Depth) + 2 ft
  • Liner Width = Pond Width + (2 × Max Depth) + 2 ft
  • Round up each dimension to the next 5-ft increment (e.g., 17′ → 20′).
Example: L=12′, W=8′, D=3′ → Length = 12+(2×3)+2 = 20′; Width = 8+6+2 = 16′ → Round to 20′ × 20′.
Trim later: Only trim after filling. Water weight pulls the liner into place.

Step 4 — Choose Your Liner: EPDM vs RPE

Fast Decision Guide

Lots of curves & corners?

EPDM 45 mil is the most flexible and forgiving for sculpted shapes.

Lightweight & long life?

RPE 30 Mil Ultra is our go-to: durable, UV-resistant, and easy to handle.

Comparison Table

Material & Thickness Flexibility Weight UV / Durability Best For Link
EPDM 45 Mil Very High Heavier Traditional choice; can tear larger when cut Complex curves & sculpted shapes Product
RPE 30 Mil Ultra Medium Light Excellent UV; long life Most backyard ponds; all-around choice Product
RPE 24 Mil Medium Very light Great UV; lighter duty Budget builds with smooth subgrade Product
RPE 20 Mil Medium Lightest Good UV Small features; gentle conditions Product
RPE 30 Mil Medium Light Excellent UV; robust General purpose ponds Product
RPE 40 Mil Ultra (Special Order) Medium-Low (stiffer) Medium Super strong High-abuse areas; structural strength Product
RPE 60 Mil Ultra (Special Order) Low (very stiff) Heavier Strongest Extreme durability; usually overkill Product

Need a box-shaped liner? We can custom-make RPE box liners. Contact us for a quote.

Step 5 — Plan Your Waterfall Liner

Safest: one continuous sheet from pond basin up the waterfall. Cost-conscious: buy a separate piece (size it generously to prevent splash-out).

Separate Waterfall Piece (Sizing)

  • Liner Length = Run length + (2 × sidewall height / max drop) + 4 ft (2 ft top & bottom)
  • Liner Width = Channel width + (2 × sidewall height) + 4 ft (2 ft each side)
  • Round up to the nearest 5-ft increment.
Leak alert: Most leaks happen in the falls from splash and mis-directed flow. Extra side material = cheap insurance.
Waterfall: measure run length, channel width, sidewall height / max drop Run Length Sidewall / Max Drop Channel Width

Step 6 — Don’t Skip Underlayment

Underlayment protects the liner from roots and rocks under the pond—and you can place it over the liner in areas where you set heavy, pointy rocks.

  • Recommendation: match underlayment size to your final liner size.
  • Product: Pond Underlayment

Quick Reference

Shape Gallons Formula Liner Formula (then round up to nearest 5-ft)
Rectangle L × W × D × 7.48 Len = L + 2D + 2   Wid = W + 2D + 2
Circle (π × (Dia/2)² × D) × 7.48 Use rectangle method with L = W = Dia
Oval / Kidney / Peanut / Lagoon (π/4 × L × W × D) × 7.48 Len = L + 2D + 2   Wid = W + 2D + 2
L-Shaped ((L1×W1×D1) + (L2×W2×D2)) × 7.48 Size each rectangle; use the larger depth for safety
Waterfall (separate piece) — Len = Run + 2×Side + 4   Wid = Channel + 2×Side + 4

Questions? We’re happy to sanity-check your numbers or quote a custom RPE box liner. Contact us.

RPE: 30 Mil Ultra · 24 Mil · 20 Mil · 30 Mil · 40 Mil Ultra (SO) · 60 Mil Ultra (SO)  |  EPDM: 45 Mil  |  Underlayment: Product