Last Updated: September 29, 2026
The honest answer to sump sizing is that no single percentage settles it, because usable volume, not nominal gallons, is what your system runs on. Working out what size aquarium sump do you need starts with the water your display sends back at power-off and the equipment your cabinet has to hold.
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By David Nguyen
How Big an Aquarium Sump You Actually Need
Size by safe working volume under normal running and at power-off, then confirm cabinet and equipment fit. There is no universal percentage, since the 10% and 25% figures openly conflict. The table gives reference points, but the real number comes from a drain-back calculation plus headroom, not from nominal volume.
| Input | Result | Note | Exception |
|---|---|---|---|
| 48×18 in tank, 1 in surface drop | 3.74 gal | Display drop component only | Excludes plumbing and overflow volume |
| 48×18 in tank, 1.5 in surface drop | 5.61 gal | Still a partial reserve figure | Overflow box adds more return water |
| Maker tank ranges | IceCap 15:10-80; 24:60-110; 30:90-160 | Cross-check after fit and safety | Range is not a required sump volume |
| BRS heuristics | ~25% vs older 10% minimum | Conflicting planning aids | Not a universal sizing rule |
Quick Answer: No Single Percentage
There is no one percentage that sizes a sump, because the common figures disagree. Bulk Reef Supply uses roughly 25% in one buyer guide and an older 10% minimum elsewhere, so treating either as a rule ignores that they are conflicting planning aids from different contexts rather than a settled requirement.
Size instead by safe working volume. The sump must hold water at power-off, run the skimmer at depth during normal operation, and fit the cabinet with the equipment, so the useful answer comes from a drain-back calculation plus headroom, then a cross-check against the maker range, not from a single ratio.
Calculate the Drain-Back Volume
The core number is the water that returns when the pump stops. Use length in inches times width in inches times the surface-drop in inches, divided by 231 cubic inches per gallon, to get the display’s contribution: a 48-by-18-inch tank dropping 1 inch yields 864 divided by 231, about 3.74 gallons.
Increase the drop and the figure rises: the same tank at a 1.5-inch drop gives 1296 divided by 231, about 5.61 gallons. These are the display surface component only and exclude the water held in the overflow box and plumbing, so add that return separately rather than treating the surface figure as the whole reserve.
How to Read the Number and Its Units
Keep the units consistent so the result is gallons. Inches times inches times inches gives cubic inches, and dividing by 231 cubic inches per gallon converts to US gallons, which is why the formula lands in the right unit; skip the division and the number is meaningless as a volume figure at all here.
Read the result as a reserve component, not a required sump size. The 3.74 and 5.61-gallon figures are how much the display sends back at those drops, so your sump must have headroom above its normal waterline to hold that plus the overflow and plumbing return, which the surface figure alone does not include.
Know the Limits of the Number
Nominal sump volume is not usable operating volume. A sump rated at a given capacity runs at a working waterline well below full, and it must keep headroom for drain-back, so the number on the box overstates what you can actually use, which is why sizing to nominal gallons misleads almost every time.
The calculation is a floor, not a full answer. It captures the display’s surface drop but not the overflow, plumbing, or the skimmer’s operating depth, so use it as one input alongside a real power-off test and the maker range rather than as a single figure that settles the sump size on its own.
Adjust for Your Situation
The reference points get you close, but your cabinet, equipment, overflow, and waterline fine-tune the real size. Use the adjustments below to move from a starting figure to a working one, since two tanks with the same display can need different sumps once fit and plumbing are taken into account.
Adjust for Cabinet and Equipment Space
Fit can cap the size before volume does. A sump must pass the cabinet door and hold your skimmer and return, so a stand that only accepts a 24-inch footprint limits capacity regardless of what a percentage suggests, which makes physical space a hard constraint on the size you can choose for your build.
Measure the interior, door, and chambers against your equipment. The IceCap 30 V2’s 13.4-by-15.4-inch skimmer chamber shows how a specific compartment governs fit, so size within what the cabinet and gear allow, since the largest sump that fits and holds your equipment is more useful than a bigger one that cannot.
Adjust for Drain-Back and Overflow
Add the overflow and plumbing return to the display figure you calculated. The 3.74 or 5.61-gallon surface component is only part of what comes back, since the overflow box and the pipes hold water too, so the sump’s headroom must cover the full return, not just the calculated surface drop alone.
Confirm the total with a power-off test. Shut the return off during setup and watch where the water settles, then adjust the working waterline until the sump holds everything safely, since the real drain-back for your plumbing is best measured directly rather than estimated from the surface calculation alone.
Adjust for the Operating Waterline
The normal running level sets both skimmer performance and reserve. IceCap V2 sumps run a 5-to-9-inch operating depth, so the waterline must sit where the skimmer runs correctly while still leaving headroom above it for drain-back, which means the usable volume is the band between those two needs.
Set the working line to satisfy both needs at once. Too high and the sump overflows at power-off; too low and the skimmer runs poorly, so the working waterline is a balance within the operating range, and the usable volume you actually get is smaller than the nominal capacity as a direct result.
Adjust Against the Maker Range
Cross-check your calculated size against the maker’s rated range last. IceCap ranges of 10 to 80, 60 to 110, and 90 to 160 gallons, or the RS-300’s 90-to-180 range with its 35-gallon sump, confirm the unit suits your display, but they are a check rather than the sizing method itself you rely on.
Use the range to validate, not to size. The maker range tells you a sump is intended for your display, while your drain-back figure, fit, and waterline decide the working volume, so let the calculation and physical checks lead and treat the rated range as the final confirmation that the choice is sound.
Frequently Asked Questions
These questions come up once the formula is clear but the tank is real. Each answer leads with the direct call, then the condition that shifts it, so you can turn a reference figure into a working sump size for your own display, cabinet, and plumbing rather than trusting a percentage.
Is There a Percentage for Sump Size?
No single one, because the common figures openly conflict. Bulk Reef Supply uses about 25% in one buyer guide and an older 10% minimum elsewhere, so neither is a universal rule. Use a percentage only as a rough starting point and size instead by drain-back, physical fit, and the maker range.
The reason is that a percentage ignores plumbing and fit. Two identical displays can need different sumps depending on overflow volume and cabinet space, so a ratio cannot capture the working volume your system actually requires, which is why the calculation and physical checks give a far more reliable answer.
How Do I Calculate Drain-Back?
Multiply the display length by the width by the surface-drop, all measured in inches, then divide by 231. A 48-by-18-inch tank dropping 1 inch gives about 3.74 gallons, and 1.5 inches gives about 5.61, which is the display surface component of what returns to the sump when the pump stops.
Then add the overflow and plumbing return, which the formula excludes. The surface figure is only part of the water that comes back, so treat it as a floor and confirm the full reserve with a power-off test, since your plumbing determines how much more arrives beyond the calculated surface drop.
Is Nominal Volume the Same as Usable Volume?
No. A sump runs at a working waterline below full and must keep headroom for drain-back, so its usable operating volume is smaller than the rated capacity. Sizing to nominal gallons overstates what you can use, which is why the box number is a poor guide to the sump you actually need.
Size to the working band instead. The usable volume sits between the skimmer’s required depth and the level that still holds drain-back, so calculate within that band rather than the full capacity, and confirm it with a power-off test to see the real operating volume your system provides once it is running.
Should I Just Buy the Biggest Sump That Fits?
Not by default, no. A larger sump can help with overall stability, but only after it fits the cabinet, holds the equipment, and reserves safely, so a unit that barely clears the door or crowds the return chamber can be worse than a right-sized one that simply works cleanly for you in daily practice.
Let fit and working volume lead. Choose the largest sump that passes every physical gate with room to set a safe waterline, rather than the biggest that squeezes in, since usable volume you can run safely matters more than nominal capacity you cannot fully use once the system is plumbed.
Final Thoughts
Size from the working volume, not a ratio: calculate drain-back as length times width times drop over 231, add the overflow return, and leave headroom, then confirm against the maker range with a power-off test. The answer to what size aquarium sump do you need is the usable volume that fits your cabinet safely.
Ready to decide? Our #1 pick for 2026 is the 48×18 in tank, 1 in surface drop.
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