Why is my tank's real volume lower than the box says?

The listed volume is usually the outside dimensions. Glass walls, a water line below the rim, substrate and hardscape all take space. Substrate is only partly lost: water fills the pores between grains, about 25% of the volume for sand and 20% for gravel in the USGS figures this calculator uses.

How do I measure the volume of rocks and wood?

Put them in a bucket of water and measure how much the level rises, or how much water you have to remove to bring it back to a marked line. That amount is the volume they displace.

Which volume should I use for fertilizer and medication doses?

The water volume after substrate and hardscape, which is what this calculator shows. Dosing by the box volume overdoses.

Aquarium Volume Calculator, After Substrate and Rock

What the calculator takes off

Glass. Two walls come off the length and width, and the bottom pane off the height. On a small tank a few millimeters of glass is a bigger share of the box than on a large one.

The gap below the rim. Most tanks are filled short of the top edge. Measure from the rim down to your water line.

Substrate, but only the solid part. A bed of sand or gravel is not solid: water fills the pores between grains. The USGS groundwater primer gives typical porosity of 25% for sand and 20% for gravel (USGS¹), so only 75% or 80% of the bed's volume displaces water. Soil and clay run much higher in the same table (55% and 50%), and we found no published porosity for aquarium soils, so for those enter a figure you measure.

Hardscape. Rock and wood displace their full volume.

Measuring the hard parts yourself

Rock and wood

Fill a bucket, mark the water line, put the hardscape in, then scoop out water with a measuring jug until the level is back at the mark. What you removed is the hardscape's volume. Dry wood floats and soaks up water, so weigh it down or measure after soaking.

Pore space of a soil

Fill a measuring jug with dry substrate to a known volume, say 1 liter. Pour water in slowly until it just reaches the top of the substrate. The water you added divided by the substrate volume is the pore space. Enter it as a percent.

Gallons and liters

US gallons here are the legal US gallon of 231 cubic inches, exactly 3.785 411 784 liters (NIST²). UK gallons are larger, so a tank sold in UK gallons will read differently.

How much water is really in the tank