How this calculator works
A sandbox is a shallow box filled with a layer of sand, so its volume is the floor area multiplied by the sand depth. The shape determines the floor area: a circle uses pi times the radius squared, a rectangle uses length times width, and a regular pentagon or hexagon uses the side length in the standard polygon area formulas. Multiplying the volume by the bulk density of the sand gives the weight. Bulk density is the mass of a loose heap, including the air gaps between grains, so it is lower than the density of solid quartz. Bags are rounded up, because you cannot buy a fraction of a bag.
Formula
round area = pi x radius^2. rectangle area = length x width. pentagon area = 1.25 x side^2 / tan(36 degrees). hexagon area = 1.5 x sqrt(3) x side^2. sand volume = area x depth. sand mass = volume x bulk density. bags = ceiling(sand mass / bag weight).
Worked example
A rectangular sandbox 2 m long and 1.5 m wide has an area of 3 m2. With 15 cm of sand for a preschool child, the volume is 0.45 m3. At 1,600 kg/m3 the sand weighs 720 kg, which is 36 bags of 20 kg. A round sandbox with a 1 m radius has an area of 3.14 m2; with 10 cm of sand it needs 0.314 m3, or about 503 kg of sand.
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Frequently asked questions
How deep should sandbox sand be?
A common guide is about 10 cm for a toddler, 15 cm for a preschool child and 20 cm for an older child who digs and builds. Deeper sand is harder to walk in and costs more; shallower sand exposes the liner and compacts quickly.
Does the calculator account for settling?
No. Loose sand settles after rain and play, so the surface drops. Buying about 10 percent extra is a common allowance, which you can approximate by raising the depth slightly.
Why does wet sand weigh so much more?
Water fills the spaces between the grains, so the same volume contains more mass. Wet sand can weigh 1,900 to 2,000 kg/m3 or more, depending on how much water drains away. The default is for dry, loose sand.
How many bags should I order?
Order the whole number shown and keep one spare bag for topping up. Sand is usually sold in 20 kg or 25 kg bags, and a 50 lb bag is about 22.7 kg; enter the bag weight you actually buy rather than a rounded guess.