About this tool
The Fish Stocking Calculator is a free, in-browser tool that estimates how heavily a tank is stocked using two classic rules of thumb side by side, then flags the result as understocked, fine, heavy, or overstocked. It is a planning guideline only — real capacity depends on filtration, water changes, species behavior and the parameters you actually test — but it catches obvious mistakes before you buy fish. Everything runs locally in your browser.
First it derates the gross volume by 15% for substrate and decor: net gallons = gross × 0.85. Method A, the inch-per-gallon rule, gives capacity_inches = net gallons × 1.0 for freshwater, or net gallons ÷ 4 for marine/reef. Method B, the surface-area method (better for oxygen exchange), uses the tank's footprint: surface area = length × width in inches, with capacity_inches = area ÷ 12 for slim-bodied freshwater fish (÷ 20 for full-bodied fish like goldfish), or ÷ 20 for marine. Your stocked total (sum of each fish's adult length × count) is compared to each capacity: under 75% is understocked, 75–100% is fine, 100–125% is heavy, and over 125% is overstocked. The verdict uses the more conservative of the two methods.
As a worked example, a 29-gallon tank measuring 30 × 12 inches has 24.65 net gallons, an inch-per-gallon capacity of about 24.7 inches, and a surface-area capacity of 360 ÷ 12 = 30 inches. Stocking 18 inches of slim fish is about 73% by the inch rule and 60% by surface area — the conservative verdict is 'understocked,' with room to add more. Always cycle the tank first and monitor ammonia, nitrite and nitrate rather than relying on any single formula.