Pond Calculator

Free pond calculators for volume, liner size, pump flow rate, filter sizing, fish stocking, treatment dosage, water change, and UV sterilizer. Essential tools for garden pond design.

Pond Calculator is a free toolkit for sizing every part of a garden pond — from how much water you'll hold to which pump, filter, and UV sterilizer will keep that water clear. Whether you're sketching a 200-gallon water garden in the backyard or planning a 6,000-gallon koi pond with a waterfall, the 15 calculators below give you the numbers before you spend a dollar on liner or pumps.

Who this site is for

  • DIY pond builders — Working from a sketch and a shovel. Use the Volume, Liner, and Edge calculators to buy the right amount of liner and stone the first time, with a 10% safety margin built in.
  • Koi keepers — Koi need 250 gallons per fish, 1–2 turnovers per hour, and roughly 10 watts of UV per 1,000 gallons. The Pump, Filter, UV, and Koi Feeding calculators turn those rules of thumb into specific GPH and wattage targets.
  • Water gardeners — Plants, goldfish, and a small waterfall. Use the Fish Stocking and Algae Risk calculators to pick a stocking level that won't tip the pond into a green-water crash in mid-summer.
  • Cold-climate pond owners — Ice and oxygen are the winter risk. The Deicer and Heater calculators give you the wattage your gallons and lowest expected air temperature actually need, not what the store clerk guesses.

How it works

  1. Pick the calculator that matches the decision you're making — sizing a new pump, ordering liner, deciding how many koi a 1,500-gallon pond can really hold.
  2. Enter your pond's dimensions in feet or meters. Every calculator supports both US (gallons, feet, GPH) and metric (liters, meters, LPH) and converts automatically when you switch.
  3. Get an answer in seconds, with the formula and assumptions shown so you can sanity-check before you buy. Add a 10–20% safety margin on pumps and filters; ponds gain stocking, plants spread, and head loss grows over time.

Why accurate sizing matters

Undersizing a pump means stagnant water and ammonia spikes that stress fish. Oversizing wastes electricity 24/7 — a 4,000 GPH pump running constantly costs roughly $30–60 more per month than a correctly sized 2,000 GPH unit. Liner that's a foot short means tearing it all out and reordering. Every calculator here uses published rules of thumb (250 gal/koi, 1× turnover/hour minimum, 10 watts UV per 1,000 gal) and shows the math so you can adjust for stocking density, climate, and waterfall height.

Popular Fish Stocking Specs

Jump straight to the gallon-by-species stocking estimate for the most common pond sizes.

Frequently Asked Questions (FAQ)

Are these calculators free to use?

Yes — all 15 calculators on this site are free, work in any modern browser, and don't require an account. There are no usage limits and no data leaves your device; calculations run locally in your browser.

US gallons or imperial gallons?

All calculations use US gallons (1 ft³ = 7.48 US gallons). If you're in the UK or Canada and need imperial gallons, multiply the US-gallon result by 0.833. The metric mode uses liters (1 m³ = 1,000 L) and is recommended outside North America.

How conservative are the rules of thumb?

Deliberately conservative. The koi stocking ratio (250 gal/koi) assumes adult fish and good filtration; smaller ponds and juvenile koi can hold more, larger koi (24"+) need 500+ gal each. The pump turnover (1× per hour) is the minimum for water gardens — koi ponds and waterfalls usually need 1.5–2×. Treat the numbers as a starting point, not a maximum.

Do I need to register or sign in?

No. Open any calculator, enter dimensions, and read the result. Bookmarks work, and you can share specific scenarios using the share button on each calculator page.