Inside, not outside

Every calculator on this site expects interior dimensions: the distance between the inner faces of the glass. Exterior measurements include two panel thicknesses in length and width and one in height. On a 10-gallon tank with 6 mm glass that is about 5 percent of the volume; on a 125 with 12 mm glass it is closer to 10 percent. If you can only reach the outside, measure the glass thickness at the rim and subtract it.

Length, width and height

  • Length runs left to right across the front. Measure near the floor where the glass is straight, not across the top trim.
  • Width (sometimes called depth) runs front to back. On bow fronts, take it at a corner for the flat depth and at the centre for the full depth.
  • Height runs from the inside floor to the rim. Do not include the stand, the bottom frame or a hood.

Fill height

Calculate to the rim and you will overstate volume by the air gap at the top. Measure from the inside floor to the surface of the water and use the fill height option. On a 12-inch-tall tank a 1.5-inch gap is an eighth of the volume.

Radius and diameter

Round tanks are calculated from the radius, which is half the diameter. Because the radius is squared, a 5 percent error in width is a 10 percent error in volume. Measure across the widest point in two directions and average them.

Curved fronts

Bow fronts need two depths: the flat depth at the corner where the curve starts and the extra bow depth at the centre. Lay a straight edge across the front from corner to corner and measure from it to the glass at the centre for the bow depth.

Corner tanks

Measure the two sides that sit against the walls, from the inside corner to where the front panel begins. Never measure the front panel itself. For corner bow fronts, also measure how far the curve bulges from a straight line between the two wall ends.

Polygon tanks

Measure a single panel between its seams. If you measure across the tank instead, divide by 1.732 for a hexagon or 2.414 for an octagon to get the side length.

Irregular shapes

Sketch the footprint from above and divide it into rectangles, triangles and arcs. Measure each piece, calculate it, and add them up. The L-shaped and trapezoid calculators cover the two most common cases.