Aquarium Sump Volume Calculator

Calculate the water volume of your sump and combine it with your display tank and refugium to find your total system volume — the figure that matters for reef dosing, salt mixing, and filtration sizing. For step-by-step measurement guidance on both the display tank and the sump itself, see how to measure aquarium volume.

System Volume Calculator

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Common Sump Sizing Guidelines

Display Tank Size Minimum Sump Recommended Sump Notes
30–55 gal 10 gal 15–20 gal Standard 20L tank fits under most stands
56–75 gal 15 gal 20–29 gal 29 gal tank is a common choice
76–120 gal 20 gal 30–40 gal Purpose-built sump recommended
121–180 gal 30 gal 40–55 gal Dual-chamber sump common
180+ gal 40 gal 55–75 gal Custom build typical

These are guidelines only. Sump size is ultimately limited by your stand dimensions. Measure your stand interior before purchasing a sump. Many hobbyists repurpose standard rectangular tanks as sumps — see aquarium volume by size for the calculated capacity of common 10, 20, and 29-gallon standards.

Why Total System Volume Matters

A reef aquarium’s total system volume includes every vessel that holds water connected to the same circulation loop — the display tank, the sump, and any refugium. When you dose a two-part supplement, add salt mix, or treat with medication, the dose must be calculated against total system volume, not just the display tank. A 75-gallon display tank with a 20-gallon sump and a 10-gallon refugium has a total system volume of 105 gallons. Dosing for only 75 gallons underdoses by 28%.

The sump serves as a reservoir that buffers against evaporation and houses equipment that would otherwise occupy display tank space. Because evaporation happens primarily from the display tank surface, the water level in the sump drops as the display evaporates — the sump acts as the visible indicator of system water loss. This means the sump’s operating water level is always below its maximum capacity, and you should measure water height to the normal operating line when calculating sump volume, not to the top of the sump walls. A sump measured to its wall height will overstate its contribution to total system volume. Display tank rock, sand, and equipment also reduce the true water figure — use the aquarium displacement calculator on the display side before summing to total system volume. For a full explanation of why total system volume matters differently in reef tanks versus freshwater, see the saltwater vs freshwater volume guide.

To calculate the display tank’s base volume before combining it with the sump, use the free aquarium volume calculator.

Sump Volume Questions

What is total aquarium system volume?
Total system volume is the combined water volume of every vessel connected to the same circulation loop — typically the display tank, sump, and refugium. It is the correct volume to use for dosing calculations, not just the display tank volume alone. A 75-gallon display with a 20-gallon sump has a total system volume of 95 gallons.
How do I measure my sump volume?
Measure the inside length, width, and water height of your sump in inches, then calculate (L × W × H) ÷ 231 for US gallons. Use the operating water level as the height — the point where water normally sits during system operation — not the maximum wall height. Sumps are typically not filled to the brim because they need space to handle return pump shutoff surge.
Why does my sump need extra capacity beyond its calculated volume?
When a return pump shuts off, water drains from the display tank through the overflow into the sump until the water level in the display drops below the overflow weir. This drain-down volume must fit in the sump without overflowing. Most sumps are sized to hold 10–15% extra capacity beyond normal operating volume to safely absorb this surge.
Should I include the refugium in my total system volume?
Yes, if the refugium is part of the same circulation loop. A refugium connected to the sump shares the same water and must be included in total system volume for accurate dosing. A separate, isolated refugium with no water exchange to the main system should not be included.
How does evaporation affect total system volume calculations?
Evaporation reduces total system volume over time by removing water without removing dissolved salts. In a reef system, evaporation is visible as a dropping water level in the sump. The rate of evaporation depends on the display tank surface area, room temperature, humidity, and whether the tank has a lid. Always top off with fresh RO water (not saltwater) to maintain the correct total system volume and salinity.
What is the sump-to-display ratio for a reef tank?
Most reef hobbyists aim for a sump that is 20–30% of the display tank volume. A 75-gallon display ideally has a 15–22 gallon sump. Larger sumps provide more water volume stability, better evaporation buffering, and more equipment space. There is no maximum — a larger sump is almost always beneficial.

Related Tools and References

Aquarium Displacement Calculator

Calculate how substrate and rock reduce your display tank's actual water volume
Aquarium Volume Glossary

Definitions of sump, refugium, overflow, evaporation rate, and total system volume
Aquarium Volume FAQ

Common questions about sump volume and total system calculations
Aquarium Volume by Size

Standard display tank sizes with calculated volumes
Aquarium Volume Unit Converter

Convert sump volume results between gallons, liters, and cubic units
Rectangular Aquarium Volume Calculator

Calculate display tank volume for standard rectangular tanks