Garden Soil Calculator

A raised bed swallows more soil than almost anyone expects. A frame that looks modest from the patio door can need sixty bags or more, and ordering once depends on both an inside-volume measurement and the volume printed on the exact bag you intend to buy. Enter the inside length, the inside width, the depth you will actually fill, the volume of one bag and a small allowance, and the result comes back as whole bags. Everything below shows where those five numbers come from and how measurement or product-label uncertainty can cross a whole-bag boundary.

Getting the bed dimensions this tool expects

1. Measure the soil cavity, not the frame. Run the tape along the inside face of one long board rather than the outside edge of the timber. A bed built from 38 mm boards loses 76 mm across each dimension, so a frame that reads 4 m outside holds 3.92 m of soil. Record both figures in meters to two decimal places. Tool: 5 m steel tape.
2. Check the width against your reach. Measure across the bed and compare it with the 4 foot adult width University of Georgia Extension recommends, which is 1.22 m. Anything wider means climbing in to weed the middle. Trim the width on paper before you calculate, not after the delivery is standing on the drive. Tool: Tape measure.
3. Set the fill depth from the base up. Hook the tape on the top of the frame and measure down to the compacted base or the drainage layer, then subtract the freeboard you want below the rim, commonly 25 to 50 mm, so watering does not wash soil onto the path. Enter what is left as the fill depth. Tool: Tape measure and straightedge.
4. Read the bag label twice. Find the net volume on the bag and convert it to cubic meters before typing it in. A 40 L bag is 0.04 m3 and a 2 cubic foot bag is 0.0566 m3. Entering 40 instead of 0.04 tells the tool that one bag holds 40 m3, and a 2.52 m3 order comes back as one bag. Tool: Bag label.
5. Decide the allowance last. Set the waste percentage only once the other four fields are correct, because it multiplies whatever is already there. The 5 percent default adds 0.12 m3 to a 2.4 m3 bed, which is three extra bags of 40 L. Raise it when the ground is uneven or the barrow run is long. Tool: None.

A four by two meter bed filled to 300 mm

The bed measures 4 m by 2 m inside the frame and is filled to 300 mm, so the soil volume is 2.4 m3. The 5 percent allowance lifts that to 2.52 m3, and dividing by the 0.04 m3 in a 40 L bag gives exactly 63. That round figure hides the rounding step this tool normally applies, which is worth seeing: switch to 50 L bags and the same 2.52 m3 needs 50.4 bags, rounded up to 51. Length, width, and depth each move raw volume directly, while bag volume moves the raw count inversely. Reducing depth 16.7 percent from 300 to 250 mm reduces the raw count to 52.5 and orders 53 bags; increasing depth 50 percent to 450 mm raises it to 94.5 and orders 95. A 2 cubic foot bag changes the denominator and orders 45. The largest real uncertainty depends on inside measurements, fill depth, and the exact bag label; at 3.92 m by 1.92 m the same bed needs 60 bags, three fewer than outside dimensions suggest.

Result: 63 bags.

Source: University of Georgia Cooperative Extension, Raised Garden Bed Dimensions.

Imperial figures are converted here at 2.54 cm to the inch and 30.48 cm to the foot, both taken from the NIST conversion card listed as source four.

Why bag volume is the input people get wrong

Four of the five fields are measurements you take yourself. Bag volume is a number you copy off a label, and labels do not agree with each other. North American bags are printed in cubic feet or in dry quarts, metric markets print liters, and this field wants cubic meters. The NIST conversion card gives 30.48 cm to the foot, so cubing that foot puts one cubic foot at 0.028317 m3. A 1 cubic foot bag is therefore 0.0283 m3, a 1.5 cubic foot bag is 0.0425 m3 and a 2 cubic foot bag is 0.0566 m3. Against the 2.52 m3 order in the worked example, those three sizes need 89, 60 and 45 bags, while a 40 L bag needs 63 and a 50 L bag needs 51. That is a spread of 44 bags of carrying for exactly the same pile of soil. Bags labeled in quarts deserve extra care, because the dry quart used for growing media is not the liquid quart on the NIST card. Take the cubic foot figure from the same label instead of converting the quart number.

Fill depth against the depth your crops actually need

The 300 mm default converts to 11.8 inches at 2.54 cm to the inch, which is a sensible place to start and a poor place to stop. University of Maryland Extension sets the minimum at 8 inches, about 203 mm, for leafy greens, beans and cucumbers, and at 12 to 24 inches, roughly 305 to 610 mm, for peppers, tomatoes and squash. University of Georgia Extension states that most garden crops need at least 10 inches, about 254 mm, of soil to thrive. So the default clears the salad threshold and lands just under the 305 mm floor for a tomato bed. Depth is also the most expensive field to guess. On the 8 m2 footprint used below, each extra 50 mm of fill is 0.4 m3 before any allowance, which is ten more bags of 40 L. Choose the crop first, set the depth from the crop, and only then run the numbers. Changing the depth after the pallet arrives is not a calculation problem.

Common mistakes

Typing the bag size in liters or cubic feet into a field that is asking for cubic meters.

A 40 L bag entered as 40 tells the tool that one bag holds 40 m3, so a 2.52 m3 order is returned as a single bag and the whole plan collapses. Divide liters by 1000 and enter 0.04, or multiply cubic feet by 0.0283 and enter 0.0566 for a 2 cubic foot bag, then sanity check that the answer is dozens rather than units.

Measuring the outside of the timber frame instead of the soil cavity inside it.

Two 38 mm boards remove 76 mm from each dimension, so a bed that reads 4 m by 2 m on the outside holds only 3.92 m by 1.92 m of soil and the order comes out three bags high. Measure inside face to inside face at the base of the bed, where boards may also have bowed outward under load, and use those two figures as the length and width.

Filling level with the top of the frame and then wondering where the soil went after the first watering.

Soil raked flush with the rim washes over the edge onto the path, and the replacement bags were never part of the calculation, so the budget and the trips both grow. Subtract 25 to 50 mm of freeboard from the frame height before entering the fill depth, then top the bed up from the compost you were going to add for the next season anyway.

Applying the waste allowance twice, once in the field and once by rounding the bag count up by hand.

The tool already rounds part bags up to the next whole bag, so adding a further 5 percent yourself turns a 63 bag order into 67 and leaves four bags to store over winter. Set the allowance once, keep it at 5 percent on a level site, and treat the returned figure as the number of bags to buy without any further padding.

Bed width, reach and why a two meter default is too wide

The 2 m default width measures 6.6 feet across. University of Georgia Extension recommends 4 feet, or 1.22 m, for adults and 3 feet, or 0.91 m, for children, because those are the widths you can work from the path without stepping on the soil you just paid to bring in. A 2 m bed is 64 percent wider than that adult figure. Narrowing it to 1.2 m rewrites the whole order: 4 m by 1.2 m by 0.3 m is 1.44 m3, which becomes 1.512 m3 with the 5 percent allowance and returns 38 bags instead of 63. The same publication sets 12 inches as the absolute minimum path between beds, 18 to 24 inches as comfortable, 4 feet where a cart has to pass and 10 feet where a truck delivers. Path widths never enter this tool, yet they decide how many beds fit the plot and therefore how many times you repeat the calculation.

Testing the ground first and when to bring in help

Volume is the easy part of a raised bed. University of Maryland Extension advises against planting vegetable crops in soil with a total lead level above 400 ppm and notes that a background level of 10 to 50 ppm is typical, so a laboratory soil test is cheaper than emptying a filled bed later. The same source puts the ideal pH between 6.2 and 6.8 for most annuals, perennials, flowers and vegetables, and organic matter at 25 to 50 percent by volume in a raised bed. On the 2.4 m3 example that means 0.6 m3 to 1.2 m3 of compost, or 15 to 30 bags of 40 L inside the total rather than on top of it. Height changes the job as well. Once the frame passes roughly 610 mm, the boards are retaining a load instead of edging a border, and University of Georgia Extension notes a vertical support about every 4 feet. At that height, get a builder to look at bracing and drainage before any soil is ordered.

Frequently asked questions

How many bags of soil does a 4 m by 2 m raised bed need?

At a 300 mm fill depth that bed holds 2.4 m3, and the 5 percent allowance takes it to 2.52 m3. In 40 L bags that is exactly 63 bags. In 2 cubic foot bags it is 45, and in 1 cubic foot bags it is 89. Narrow the bed to 1.2 m so you can reach the middle from the path and the same depth needs only 38 bags of 40 L.

Should I enter the frame height or the soil depth?

Enter the soil depth. The frame height also covers the freeboard you leave below the rim and any drainage layer at the base, and neither of those is bagged soil. On an 8 m2 bed a 50 mm difference between frame height and fill depth is 0.4 m3, which is ten bags of 40 L, so the distinction is worth a second measurement rather than an estimate.

Is a 300 mm fill depth enough for tomatoes?

It is close, but short. University of Maryland Extension gives 12 to 24 inches, or 305 to 610 mm, for peppers, tomatoes and squash, against 8 inches or 203 mm for leafy greens, beans and cucumbers. University of Georgia Extension sets a general floor of 10 inches, or 254 mm. Raising the example bed from 300 mm to 400 mm takes the order from 63 bags to 84.

When does a bulk load beat buying bags?

Sixty three bags of 40 L is 2.52 m3, which the NIST conversion card puts at about 3.3 cubic yards using 0.76 m3 per cubic yard. The choice above one or two cubic meters is usually about handling rather than volume. A loose load lands in one heap and has to be barrowed along the paths, while bags can be carried to each bed but leave plastic to dispose of afterwards.

How much of the fill should be compost?

University of Maryland Extension puts organic matter at 25 to 50 percent by volume in a raised bed and suggests a 1:2 or 1:1 mixture of compost and purchased topsoil. On the 2.4 m3 example that is 0.6 m3 to 1.2 m3 of compost, or 15 to 30 bags of 40 L within the 63. For a bed already in use, the US EPA advises mixing two to four inches of compost into the top six to nine inches of soil, and a 50 mm layer over 8 m2 comes to eleven bags once the allowance is applied.

Related calculators

• Lumber Calculator The boards that form the bed sides set the inside dimensions you enter here, so size the frame timber before you fix the soil volume.
• Mulch Calculator Mulch is spread in a thin layer over the filled bed rather than through it, so it is priced by area and depth instead of the full cavity volume.
• Gravel Calculator A drainage layer under the soil or a gravel path around the bed comes out of a separate volume that must not be counted as fill depth.
• Concrete Calculator Beds that sit on pads or need footings under the corner posts add a small pour, and that volume is worked out from depth and area the same way.
• Mortar Calculator A block or brick bed wall replaces the timber frame, so the bed volume stays the same while the wall itself needs a bedding and jointing quantity.
• Paint Calculator Only the outer faces of a timber bed are worth finishing, so measure the exposed sides and ends rather than the whole board area you bought.

Sources

1. Raised Garden Bed Dimensions University of Georgia Cooperative Extension. Supplies every figure in the dimensions table: the 4 foot and 3 foot bed widths, the 10 inch minimum soil depth, the 24 and 36 inch bed heights, the 12 inch to 10 foot path widths, the 12 foot length limit and the support spacing about every 4 feet. Accessed 2026-09-07.
2. Soil to Fill Raised Beds University of Maryland Extension. Source of the 8 inch and 12 to 24 inch crop depths, the 25 to 50 percent organic matter range by volume, the 6.2 to 6.8 pH band, the 1:2 and 1:1 compost mixes and the 400 ppm lead limit quoted in the sections and the questions. Accessed 2026-09-07.
3. Composting at Home United States Environmental Protection Agency. Backs the two to four inches of compost worked into the top six to nine inches of soil, which sets the annual top up volume of eleven bags quoted in the questions. Accessed 2026-09-07.
4. Approximate Conversions from U.S. Customary Measures to Metric NIST Office of Weights and Measures. Provides the 2.54 cm inch, the 30.48 cm foot and the 0.76 m3 cubic yard used to turn bag labels, crop depths and bulk loads into the cubic meters this field expects. Accessed 2026-09-07.

Prepared and maintained by Murat Yilmaz · Calculations checked against the cited sources and reference tests · Updated September 7, 2026

This is a planning result, not a supplier quote; sellers use different pack, bag, board, roll, and bundle conventions, so confirm the exact product label and selling unit before you order.