Soil Calculator

Bags = length (ft) × width (ft) × depth (in) ÷ 12 ÷ 1.5 ft³ per bag

L

Length of the bed or planter box

W

Width of the bed or planter box

D

Depth of soil you are filling, in inches

b

Volume of one bag — 1.5 cubic feet in the count above

Use it when you are filling boxes rather than dressing ground. A raised bed needs the depth the crop needs, not the depth of the box: lettuce and herbs are happy at 6 inches, carrots and tomatoes want 10 to 12, and filling a 12 inch box to the top with soil is a common way to spend twice what the crop needs. Measure the inside of the box rather than the outside, and if the bed is already part full, fill in only the depth you are adding.

1. An 8 by 4 ft raised bed filled 10 inches deep with topsoil

   1. Volume: 8 × 4 × (10 ÷ 12) = 26.67 ft³
   2. Bags: 26.67 ÷ 1.5 = 17.8, rounded up to 18
   3. In metric: 26.67 ft³ = 755 L
   4. Weight at 1,280 kg/m³: 967 kg, or 2,131 lb

   Eighteen bags is a car load, and at 967 kg it is not a trip you want to make twice. Bagged soil is sold by volume, so check whether the pallet price beats the per-bag price before you start carrying — the volume is the same either way.

2. A 12 by 4 ft bed, 12 inches deep, filled with garden mix

   1. Volume: 12 × 4 × (12 ÷ 12) = 48 ft³
   2. Bags: 48 ÷ 1.5 = 32 exactly
   3. In metric: 48 ft³ = 1,359 L
   4. Weight at 1,100 kg/m³: 1,495 kg, or 3,296 lb

   Garden mix is lighter than topsoil — 1,100 kg/m³ against 1,280 — so the same 48 ft³ weighs about 250 kg less. That matters on a roof terrace or a balcony, where the weight rather than the volume is what the structure has to carry.

3. A small 4 by 4 ft planter box, 6 inches of compost

   1. Volume: 4 × 4 × (6 ÷ 12) = 8 ft³
   2. Bags: 8 ÷ 1.5 = 5.3, rounded up to 6
   3. In metric: 8 ft³ = 227 L
   4. Weight at 720 kg/m³: 163 kg, or 360 lb

   Compost is the lightest of the three at 720 kg/m³ and the one to mix rather than fill with — a box of pure compost holds water and slumps as it decomposes. Most people blend it a third at a time into topsoil, in which case calculate the volume with this page and split it between the two materials.

The bag count assumes a 1.5 cubic foot bag. That is the most common size for bagged raised bed soil in the United States, but 2 cubic foot bags are widely sold and some soil comes in 40 lb bags sold by weight — so treat the count as a figure to scale rather than a fixed number, and divide by the bag size you are actually buying. The materials are given typical densities (±20%), not specifications: a compost that has broken down further, or a topsoil that is wet rather than dry, will weigh noticeably more or less than the figures here, and the volume is unaffected. The densities are also what the bag weighs when it is damp, which is how it arrives; a dry bag weighs less. Depth is the soil you are adding, not the height of the box, and if the bed has gravel or rubble in the bottom, measure from the top of that. Nothing here accounts for settling: fresh bagged soil drops a few per cent in the first season, and a box filled level in spring will look low by autumn.

How many bags of soil do I need for a raised bed?

Multiply length by width by depth to get the volume, then divide by the bag size. An 8 by 4 foot bed filled 10 inches deep is 26.7 cubic feet, which is 18 bags at 1.5 cubic feet each. Switch the bag size if yours are 2 cubic feet: the same bed then takes 14.

How deep should the soil in a raised bed be?

Six inches is enough for lettuce, herbs, onions and radishes. Ten to twelve inches is what carrots, tomatoes, peppers and most root crops want. Twelve inches is also the practical limit for most raised beds — beyond that the volume and the weight both climb faster than the crop improves.

Should I fill a raised bed with topsoil or with a mix?

Neither pure topsoil nor pure compost. Topsoil alone is heavy and short of organic matter; compost alone holds too much water and slumps as it breaks down. A common blend is roughly half topsoil, a third compost and the rest garden mix or aged manure, which is close in density to the garden mix figure on this page.

How much does a bag of soil weigh?

A 1.5 cubic foot bag of topsoil weighs roughly 120 lb, and the same bag of compost roughly 67 lb, because compost is far lighter per unit volume — 720 kg/m³ against 1,280. Bags are sold by volume but carried by weight, and a balcony or roof terrace has a limit on the second one.

Can I use this if my bags are a different size?

Yes, but scale the answer. The count above is for 1.5 cubic foot bags; for a 2 cubic foot bag multiply the volume in cubic feet by 0.75, and for a 50 litre bag work in litres. The volume in cubic feet and litres is on the result panel too, so the bag count is never the only thing you can take to the shop.

Should I buy bags or have bulk soil delivered?

A raised bed is a bag job: 18 bags fit in a car and can be carried through a side gate. A lawn or a long border is not — at 200 bags the bulk price by the cubic yard is far lower, and no one carries that many bags from a car park. The dividing line is roughly a cubic yard, which is about 18 of the 1.5 cubic foot bags.

• Guide for the Use of the International System of Units (SI), NIST Special Publication 811 — the cubic metre, the litre and the kilogram behind the metric figures — National Institute of Standards and Technology
• The International System of Units (SI) brochure — the litre, which is where the metric volume on this page comes from — Bureau International des Poids et Mesures (BIPM)