Free aquarium calculators — freshwater, planted & reef

Set up your tank once.
Every tool knows it.

Measure your tank, pick your water, and every calculator on the site runs on your volume — water changes, CO2, fertilizer, salt mix, alk and calcium dosing, filters and pumps. Standard formulas, gallons or liters, math shown.

Open my dashboard Browse all calculators

Published formulas, cited
Remembers your tank
Gallons & liters, no login

1

Set up your tank

Dimensions or volume, water type, sump if you have one. Two minutes, once, saved on your device.
2

Pick a tool

The tools for your kind of tank come first. Every one starts already filled in with your volume.
3

Test, enter, dose

The dashboard takes this week's readings and turns them into what to do — with the math shown.

The flagship

One dashboard, three faces

The dashboard changes shape with your water type. A reef keeper never sees a CO2 chart; a planted-tank keeper never sees alkalinity dosing. Same tank profile underneath, the right questions on top.

Freshwater Ammonia, nitrite, nitrate, pH → cycle status, water-change percentage, next change.
Planted All of that, plus pH + KH → dissolved CO2, and this week's dry-salt fertilizer doses.
Reef Salinity, alk, Ca, Mg → daily two-part doses, or the water change that fixes it instead.

Open my dashboard →

Tank sheet · example "75 gallon" · 48 × 18 × 21 in · 2 in gravel · filled 1 in below the rim

Volume to the rim: 78.5 gal · 297 L
Water at the fill line: 74.8 gal · 283 L
Gravel bed: 99.9 lb · 45.3 kg, displaces 4.5 gal · 17 L
True water volume: 70.3 gal · 266 L — 6% under the label, 10% under the rim
Water weight: 585 lb · 265 kg fresh · 601 lb · 273 kg salt
Turnover target: 280–420 GPH freshwater · 1,400–2,800 GPH reef

Every number here is computed by the same engine the calculators use — nothing is typed in. Dosing tools use the true volume: dose "75 gallons" of conditioner into this tank and you're overdosing by 7%.

The toolbox

Every calculator, grouped the way you keep fish

Set your tank once — every tool below reads it. Each one shows its formula and its source, and takes gallons or liters without asking.

Setup & Volume

The numbers everything else is built on.

Tank Volume Rectangle, bowfront, cylinder, hex or cube — gallons and liters, to the rim or to your fill line. Aquarium Weight Water, glass, substrate and rock — the "will my floor hold it" number. Substrate & Sand Footprint and depth → pounds or kilos of gravel, sand or aquasoil, and how many bags. True Water Volume What your tank really holds after rock and substrate — the volume every dose should use. Glass Thickness Pane thickness from the plate formula with its coefficients shown, the safety factor your tank was really built to, and what a brace buys. Heater Wattage Watts from real heat loss — your glass, your lid, your coldest room — plus how hot a stuck-on heater gets and what it costs to run. Electricity Cost What your tank costs to run per month — lights and pumps by the hour, the heater from real heat loss, at your rate. Power Outage How many hours your tank stays warm without power, what a UPS or battery really runs and for how long, and whether the sump can swallow the drain-down. Stocking Level Your fish list as the nitrate it makes and the water change that holds it — with schooling, length, water and temperament as flags, not a mystery percentage.

Freshwater & Planted

Cycling, water changes, hardness and CO2.

Water Change Current → target nitrate: what percentage to change now, or how many changes a week. Water Conditioner Dose "How much Prime for 10 gallons?" — dechlorinator doses for any product and any volume. Fishless Cycling Ammonia Milliliters of ammonia solution to hit 2–4 ppm, and the re-dose schedule. Free Ammonia "0.25 ppm — is that dangerous?" Your kit reads total ammonia; this splits it into the toxic NH₃ by pH, temperature and salinity, grades it, and sizes the water change. CO2 Calculator pH + KH → dissolved CO2 in ppm with its real error bar, the pH-drop method, and what your drop checker color means. CO2 Cylinder Duration How many days your cylinder lasts at your bubble count — with what one bubble really holds stated, not hidden — your own bubble from the last refill, the empty date, and DIY citric acid or yeast yields. Fertilizer Dosing (EI) Dry-salt doses for EI, half EI, PPS-Pro or your own targets — grams, stock bottles, and the level the tank really settles at. GH / KH Adjustment Baking soda to raise KH, gypsum and Epsom at a real Ca:Mg ratio to raise GH, or the RO blend that lowers either. Hardness Converter dGH ↔ ppm ↔ meq/L ↔ gpg ↔ °Clark — and GH from the calcium and magnesium on a water report. Quarantine Length Start date + tank temperature → the date the quarantine clears, as complete parasite life cycles rather than a number of weeks somebody quoted.

Saltwater & Reef

Salinity, alk, calcium, magnesium — the weekly ritual.

Two-Part Dosing Consumption from two test points → daily alk and calcium doses for your products. Alkalinity Dosing Soda ash, baking soda, kalkwasser or commercial — grams or mL to hit your dKH. Calcium Dosing Calcium chloride (dihydrate or anhydrous) or commercial, to your target ppm. Magnesium Dosing MgCl2 + MgSO4 blend ratio and dose to your target. Salt Mix Grams or cups of salt for X gallons at target salinity, per brand. Salinity Adjustment SG is 1.028 — how much RODI to add to reach 1.025? And the reverse. Salinity Converter Specific gravity ↔ ppt ↔ conductivity, temperature-corrected. Alk Unit Converter dKH ↔ meq/L ↔ ppm CaCO3. Water Change Correction Fix a parameter by water change instead of dosing — and which is saner. Evaporation & ATO Sizing Days of autonomy for a top-off reservoir, with kalk-in-ATO option. DI Resin Life & RO/DI Cost Gallons per DI cartridge from your tap TDS and membrane, how many days that is at your usage, and what a gallon of RO/DI really costs all-in.

Equipment Sizing

Buy the right size the first time.

Filter Size What size filter for your tank — the rating to shop for, what it really moves with media in, and the media volume that is the actual spec. Return Pump Sizing The flow a pump really delivers against your lift, pipe and fittings — from its curve, not a rule of thumb — and whether the drain can take it. Flow & Powerheads Circulation by what you keep — softies to SPS — with the return pump counted, split across the powerheads your tank wants, and what it costs in watts. Chiller Sizing Heat load from your glass, pumps and lights minus evaporation → BTU/h and hp — and what a fan alone would hold the tank at. UV Sterilizer Sizing Dose from lamp watts and flow — green water, bacteria or parasites — the lamp your tank needs, what age and clarity cost, and what UV can't do.

Two hobbies, one toolbox

Which tank do you keep?

Freshwater and reef keeping share the math of volume and share almost nothing else. Pick your water in the profile and the site puts your tools first — the others stay one click away for the second tank.

Freshwater Cycling, water changes, stocking, hardness. The dashboard reads ammonia, nitrite, nitrate and pH and tells you what to change this week. Freshwater tools → Planted Everything freshwater plus CO2 from pH and KH, and dry-salt fertilizer doses for EI or lean regimes — the Rotala-Butterfly job, done cleanly. Planted tools → Saltwater & reef Salt mix, salinity, alkalinity, calcium and magnesium. Enter this week’s alk/Ca/Mg tests and the dashboard hands you all three doses. Reef tools →

Guides

The why behind the numbers

Short, honest guides that pair with the calculators — what each number means, how to move it, and what the store won't tell you.

The nitrogen cycle explained Ammonia → nitrite → nitrate, why it takes weeks, and how to know when it is done. Why "one inch per gallon" is wrong The rule everyone quotes and nobody should use — and what to use instead. Freshwater parameters chart Every target range on one printable page: pH, GH, KH, ammonia, nitrite, nitrate, temp. Cloudy water: causes and fixes White, green or gray — each one is a different problem with a different fix. Reef parameters chart Alk, Ca, Mg, salinity, nutrients — targets by coral type, on one printable page. RODI water: why tap water fails What is in tap water, what it does to a reef, and what a system really costs to run.

Math you can check

Every result comes from a published formula or a cited reference — named on the page, with a “show the math” breakdown using your own numbers.

Gallons and liters, equally

Liters, centimeters and dKH aren't a toggle we bolted on. Every tool takes either, remembers which you use, and formats results the way you'd say them.

Private by design

No accounts, no tracking dashboards. Your tank profile and test log live in your browser and nowhere else.

Honest recommendations

When a result suggests gear, it's the piece the numbers justify — and we say so when the cheap option wins. Some links earn a commission; that never changes the math.

Set up your tank once.Every tool knows it.

Set up your tank

Pick a tool

Test, enter, dose