Formula
A rectangle is length × width × water height. A cylinder uses the length as the diameter. A regular hexagon has area (3√3 / 2) × side², times the height. A bow-front is the rectangle plus a parabolic segment, (2/3) × the front chord × the bow depth, times the height. That segment is what you use when you do not have the glass radius.
gallons = cubic inches / 231
fresh weight = gallons × 8.34 lb
salt weight = gallons × 8.55 lb
8.55 is a planning salt-water weight, not a salinity calculation. The stand has to carry the water plus the glass, the stand, and whatever is in the tank. This number is only the water. Pool gallons, in feet rather than inches, are the pool volume calculator.
Worked examples
48 × 13 × 20 in, fresh. 12,480 in³ / 231 = 54.03 gallons. At 8.34 lb/gal that is 450.6 lb, about 204 kg, of water.
A hexagon with 12 in sides and 12 in of water. Area (3√3 / 2) × 144, times 12. Divide by 231 for gallons.
The same rectangle marked salt water. The gallons do not change. The weight uses 8.55 instead of 8.34.
Reference
Weight factors. The stand load below is water only.
| Water | lb/gal |
|---|---|
| Fresh | 8.34 |
| Salt, planning | 8.55 |
Questions
Is the bow-front exact?
No. Without the glass radius, the added water is a parabolic segment, two thirds of the chord times the bow depth. A radius from the manufacturer replaces that.
Does the weight include the glass and the stand?
No. It is the water only, at 8.34 lb per gallon fresh or 8.55 lb per gallon if you mark it salt water. Rock, sand, and the tank itself are extra.
Why divide by 231?
A US gallon is 231 cubic inches. A 48 × 13 × 20 inch tank is 12,480 cubic inches, which is 54.03 gallons.
Sources
- 1 US gallon = 231 in³. Fresh water at 8.34 lb/gal, the same figure the salt page uses.
- Regular hexagon area = (3√3 / 2) × side². The bow-front segment (2/3) × chord × bow depth is a parabolic approximation, used because the glass radius is usually unknown.
- 8.55 lb/gal is a planning salt-water weight for this page, not a salinity model.