Work out your tank's real water volume after substrate and rock, then calculate water changes, dosing, and heater size.
Assumes 10-litre buckets. Adjust for your actual bucket size.
An aquarium's rated volume is its gross capacity. When you add substrate (sand or gravel) and hardscape (rocks, driftwood), they displace water. A 100-litre tank with 10 litres of substrate and 5 litres of rock holds only 85 litres of actual water. This matters because you dose medications, fertilizers, and test kits based on real water volume, not the box's promise.
Most keepers discover this too late: they overdose ferts because they calculated by the sticker on the glass, or their heater can't maintain temperature in winter, or water change math is off. Knowing your usable volume is the first step to keeping fish healthy.
Fill height matters too. A 60-litre tank (60 x 30 x 40cm) filled to only 33cm instead of 40cm loses 4.2 litres to that gap at the top, which is needed for filter turnover and surface agitation.
| Substrate type | Depth | Volume displaced |
|---|---|---|
| Fine sand 1 inch of aquarium sand (ADA Aquasoil, Flourite). | 1 in | 5% |
| Medium gravel 2 inches of typical aquarium gravel or planted substrate. | 2 in | 10% |
| Coarse gravel 3 inches of larger pea gravel or river rock bedding. | 3 in | 15% |
| Rock hardscape Significant rocks, driftwood, or aquascaping décor without live plants. | N/A | 5-20% |
| Densely planted Plants, gravel, and wood occupy substantial space; some areas hollow. | Variable | 15-25% |
Estimates based on typical grain size and packing. Finer substrates settle denser; larger grains pack more loosely. Rock and driftwood volume depends on aquascaping style, not a fixed rule. Measure the volume directly if you need precision, or use the percentages as a reasonable guide.
| System type | Weekly % | Biweekly % |
|---|---|---|
| Lightly stocked planted Low bioload, CO2, fertilizers manage chemistry. Can skip weekly. | 15% | 25-30% |
| Typical community tank Balanced fish load, some plants. Moderate bioload from food waste. | 25-30% | 40-50% |
| Heavily stocked or goldfish High bioload. Fish produce lots of ammonia and waste. Nitrate climbs fast. | 40-50% | 60-75% |
| Saltwater reef Water quality parameter swings are costly; smaller changes maintain stability. | 10-25% | 20-30% |
Water change percentages are rules of thumb based on aquarium load and filtration. The real guide is your test kit: if nitrate stays under 40 ppm in freshwater and alkalinity stable in reef, your frequency is right. Increase changes if nitrate spikes or pH drifts.
The baseline is 3 watts per gallon for every 10 degrees Fahrenheit of temperature rise. A 50-gallon tank needing an 8-degree rise (68 F room to 76 F water) needs 50 × (3 × 8/10) = 120 watts. This assumes a typical lid, room air circulation, and no major heat loss.
Adjust the multiplier for your setup:
| Tank size / lid type | Watts per gallon |
|---|---|
| Small tanks (under 20 gal), open top More surface area per litre, heat loss is higher. | 4-5 W/gal |
| Medium tanks (20-60 gal), standard hood The baseline. Typical aquarium conditions. | 3-3.5 W/gal |
| Large tanks (over 60 gal), tight hood Less surface area per litre; water mass provides thermal inertia. | 2.5-3 W/gal |
| Cold room (below 65 F ambient) Increase the baseline by 25-50%. | +25% to +50% |
The calculator uses 3 W/gal, a safe middle ground. If your tank is in a drafty room or you want redundancy (two smaller heaters instead of one), size up. If your room is warm year-round, you may size down.
Method 1: Inside dimensions. Measure the inside (glass-to-glass) length, width, and height in centimetres. Multiply them: cm × cm × cm / 1000 = litres. Most tanks are rectangular; the calculator handles this. If your tank is bow-front, hexagon, or cylinder, you will need a different calculator or to estimate an equivalent rectangular volume.
Method 2: Fill and measure. Fill the tank to your normal operating level. If you have a pump or powerhead that pulls water, mark the level on the glass with tape. Empty the tank into a graduated bucket and count litres, or use a flow meter on the drain. This is the gold standard for accuracy but tedious for a 100-gallon tank.
Method 3: Water change tracking. Do a large water change (50%) and note how many buckets you removed. Multiply buckets by their volume: if you removed 5 ten-litre buckets, your usable volume is roughly 100 litres. This gives you real usable volume, automatically accounting for substrate and décor.
Measure the inside dimensions of your tank in length, width, and height. Multiply them together and divide by 1000 (metric) to get litres, or use the calculator above. Then subtract the volume displaced by substrate, rocks, and decorations. Most substrates and rock take up 5-15% of the tank's total volume.
Most freshwater aquariums benefit from a 20-30% weekly water change. Heavily stocked tanks may need 40-50% per week. Planted tanks with good filtration can get away with 15-20% biweekly. The calculator uses 25% as a sensible default, but adjust based on your fish load and how quickly nitrate builds up.
Use the formula: Watts = Gallons x (3 watts per gallon) x (temperature rise in degrees Fahrenheit / 10). For a 50-gallon tank, room at 68 F, target water 76 F (8 degree rise): 50 x 3 x (8/10) = 120 watts. Smaller tanks need more watts per gallon; larger tanks need fewer.
Substrate (sand or gravel) is solid material that takes up space. A 2-inch layer of sand in a 100-litre tank occupies real volume. When you add this substrate, the actual water capacity shrinks. You must account for this displacement to know the true volume for dosing medications, calculating water changes, or sizing filters.
Yes. Most keepers do not fill their tank to the very rim; there is usually 1-2 inches of empty space to prevent overflow during filter turnover and water surface movement. Enter your actual fill height, not the tank's total height.