Garden Bed Calculator

How to estimate a whole bed

A bed is three purchases from one measurement. Topsoil and compost go in as a mix, mulch goes on top afterwards, and each has its own depth. A soil supplier will size the first two and a bark supplier the third, and neither will mention the other.

fill (cu ft) = bed area × fill depth (in) ÷ 12
topsoil = fill × (1 − compost share)   compost = fill × compost share
mulch (cu ft) = bed area × mulch depth (in) ÷ 12Mix ratio from UMN Extension. Mulch depth from UMass Extension and the University of Florida's Florida-Friendly Landscaping program.

Measure the bed once

Inside the frame, not outside. A bed built from 2-inch lumber loses four inches across each direction, and on a 4 by 8 bed that is about 10 percent of the volume you would otherwise buy.

For a ground-level bed with no frame, measure the planted area as you would walk it. A curved edge can be averaged into rectangles; being off by a foot changes the answer by less than a bag.

Two depths, and they are not the same number

This is where a single-material calculator leaves you guessing.

Fill depth is how much soil mix goes in. Most raised beds are 10 to 12 inches, and you want to stop an inch or two below the rim so the mulch has somewhere to sit. If the frame is 12 inches tall, filling to 10 and mulching 2 brings you level.

Mulch depth is two to three inches, and that range is not a guess. UMass Extension recommends 2 to 3 inches and adds "never more". The University of Florida's Florida-Friendly Landscaping program gives the same range and says to top up to maintain it.

Deeper mulch is actively worse rather than merely wasteful. Piled against stems it holds water where the plant does not want it, and a thick mat can shed rain instead of letting it through. Four inches is defensible where weed suppression is the whole point and nothing is planted close, but it is the ceiling.

Fill to the top and then mulch on top of that, and the first watering puts some of both on the path.

The mix, and why it is two bag counts

A raised bed is not filled with topsoil alone. UMN Extension states the ideal mixture as around a half to two thirds topsoil and a third to a half compost.

That changes what you buy rather than how much. The total volume is the same either way; what moves is the split, and those are two different products on two different pallets. At 26.7 cubic feet and a third compost you want about 17.8 cubic feet of topsoil and 8.9 of compost, which is 18 bags and 9 bags at a cubic foot each. This page counts them separately because that is how you will be standing at the register.

Pre-mixed garden soil is the same two things already blended. It costs more per cubic foot and saves the mixing, and the volume you need does not change.

Mulch is optional, and there is one case where it is wrong

The mulch depth has a None option for a reason. Mulch around established plants holds moisture and suppresses weeds, and that is as true in a vegetable bed as an ornamental one.

What it does not suit is a bed you are about to direct-sow. A carrot seedling has to get through whatever is on top of it, and two inches of bark is a wall. Sow first, let things come up, then mulch between them.

Bags or bulk

A cubic yard is 27 cubic feet. Soil is commonly bagged at 0.75 or 1 cubic foot and mulch at 2, so the same volume is far more bags of soil than of mulch, and each one is heavier: a cubic foot of damp topsoil runs about 40 pounds.

Under a cubic yard, bags win. They fit in a car, you can store the extras, and you open only what you need. Past about two cubic yards you are opening fifty or more, and bulk delivery is usually cheaper per yard and far less work. That crossover is a handling judgment rather than a spec, so price both.

One caution on bulk for a single bed: a yard of soil is a much bigger pile than it sounds and weighs roughly a ton. It has to be dumped somewhere you can work from.

A quick cost check

The 4 by 8 bed, filled 10 inches with a third compost, mulched 3 inches. That is 18 bags of topsoil and 9 of compost at a cubic foot each, plus 4 bags of mulch at 2 cubic feet. At $5 a bag for soil and compost and $4 for mulch, that is $135 plus $16, so about $151 for one finished bed.

Build three of them and you are at 81 bags of soil and compost, which is where a bulk delivery starts making sense on both price and on the number of trips.

Common estimating mistakes

• Measuring the outside of the frame. On a 4 by 8 bed built from 2-inch lumber the inside is about 3 ft 8 in by 7 ft 8 in.
• Using one depth for both materials. Fill and mulch are different numbers over the same area.
• Buying it all as topsoil. The compost share is a real part of the order and a separate pickup.
• Mulching a bed you are about to sow. Set the mulch to None and come back to it.
• Filling flush to the rim. Leave room for the mulch, or it ends up on the path.
• Pricing in a settling factor. Nobody publishes one worth using. Fill, water, wait a week, top up.

When to hand it off

Filling beds is not skilled work, so the honest boundary here is weight. A cubic yard of soil is about a ton, and three beds is several. Past three or four yards, pay for delivery to the closest point you can reach and get help carrying it.

The thing worth handing off is diagnosis. If ground stays wet, grows nothing, or drops year after year, more soil on top treats a symptom. Drainage and grading are their own trades, and a raised bed built over a drainage problem is still sitting on a drainage problem.