Loan Tank Volume Estimator
Gallons are easy to ask for and surprisingly slippery to compute — and the weight hiding inside those gallons is what floors and stands actually care about. Pick the shape, enter the dimensions: get US gallons, litres, full weight, and substrate bags for aquariums.
The defaults are the classic 55-gallon aquarium footprint (48 × 13 × 21 in). Use inside dimensions where you have them — glass thickness quietly shaves a gallon or two off the brim figure.
How it works
Everything runs through cubic inches, then divides by 231 cu in per US gallon. Rectangular: L × W × H. Round: π × (diameter/2)² × height. Water weighs 8.34 lb per US gallon, so the classic 55-gallon aquarium is a 473-lb decision before gravel, glass and stand — the weight line is the one to read twice. Substrate for aquariums plans at roughly one 20-lb bag per 10 gallons for a planted-depth bed.
Formula & worked example
Formula: Gallons = cubic inches ÷ 231. Rectangular: L × W × H; round: π × (diameter ÷ 2)² × height. Litres = gallons × 3.78541; water weight = gallons × 8.34 lb. Aquarium substrate plans at one 20-lb bag per 10 gallons.
Worked example: The classic 55-gallon footprint, 48 × 13 × 21 in: 13,104 cu in = 56.73 US gallons, 215 litres, 473 lb full of water, about 6 substrate bags.
When to use this
- You are setting up an aquarium and the stand/floor needs the full weight.
- You are sizing a rain barrel, stock tank or cistern by dimensions.
- You are dosing water treatment and need true gallons, not the marketing size.
How to use it, step by step
- Pick the shape — rectangular or round.
- Enter inside dimensions in inches.
- Read gallons and litres for capacity decisions.
- Read the water weight before choosing the stand or the room.
What the inputs mean
| Input | What it is | Typical values |
|---|---|---|
| Dimensions | Inside length, width, height (or diameter) | 48 × 13 × 21 in classic 55-gal footprint |
| Shape | Rectangular or round (cylinder) | Aquariums rect; barrels and stock tanks round |
Another worked example
A round stock tank, 24 in diameter and 30 in tall: π × 12² × 30 = 13,572 cu in = 58.75 US gallons (222 litres), weighing about 490 lb full. Doubling the diameter to 48 in would quadruple it to roughly 235 gallons — round volumes grow with the square of the diameter.
Frequently asked questions
How many gallons is a 48 × 13 × 21 in tank?
56.73 US gallons at the brim (13,104 cu in ÷ 231) — the classic "55 gallon" aquarium, which in practice holds about 51 usable gallons once substrate, decor and a sane waterline are in. Full of water it weighs roughly 473 lb, and a planted substrate bed takes about 6 bags.
How many gallons is a 24-inch round tank, 30 inches tall?
About 58.75 gallons: π × 12² × 30 = 13,572 cu in. Round tanks hide volume well — doubling the diameter quadruples the gallons, which catches people sizing stock tanks and rain barrels.
Why show the weight?
Because water is the heaviest thing most homes hold. A 75-gallon tank is 626 lb of water alone on four small feet. Second floors, old joists and particleboard stands all have opinions about that — check before filling, not after.
How much substrate do I need?
Plan on about one 20-lb bag per 10 gallons for a 1–2 in bed: 6 bags for the 55-gallon footprint above. Deep planted beds (3 in+) want half as much again; bare-bottom and breeding tanks want none.
How do I size a rain barrel or cistern?
Work the roof backwards: 1 inch of rain on 1,000 sq ft of roof yields about 623 gallons. A 55-gallon barrel fills from a surprisingly small storm — most cistern regrets are about too little storage, not too much.
🚿 What does all that water cost?
Storage sized, the Water Bill Estimator builds your household's monthly and yearly water + sewer bill fixture by fixture, at your local rate.
Why is my '55 gallon' tank only 51 usable gallons?
Brim volume minus an inch of freeboard, substrate displacement and decor. Working volume is typically 90–95% of brim for aquariums; filters and dosing should use the working figure.
Do I measure inside or outside dimensions?
Inside for true volume. A 48-in outside tank with ½-in glass is a 47-in tank inside — about 2% less volume, and the difference matters for medication dosing more than for bragging.