How do you calculate topsoil, mulch and turf quantities for a landscape job?

What is the formula for soil and mulch volume?

The formula for soil and mulch volume is volume (m3) = area (m2) x depth (m). Every topsoil, planting mix and mulch quantity in a landscape takeoff comes back to that one line, so the accuracy of the result depends on the accuracy of the area and the depth you feed into it.

Depths on landscape drawings and specifications are almost always given in millimetres, so convert them to metres before multiplying. To convert millimetres to metres, divide by 1,000: 300 mm is 0.3 m, 150 mm is 0.15 m and 75 mm is 0.075 m.

The unit conversion is where the most expensive errors hide. Dividing by 100 instead of 1,000 makes the quantity ten times too large, and skipping the conversion entirely makes it a thousand times too large. A quick sense check against the size of the job catches both.

Where does the depth come from?

The depth comes from the specification and the construction details, not from habit. A planting bed detail will usually show the depth of planting mix, any subsoil layer and the mulch on top, and the specification will name the product for each layer. That is the depth you measure to, even if it differs from what you normally lay.

Check that the details and the specification agree. Where one says a different depth to the other, or where a depth is not given at all, raise it as a tender query rather than assuming. Our guide to reading a landscape tender scope covers how to find and flag these gaps.

How do you work through a soil calculation?

A soil calculation is a short, repeatable process. The figures below are made-up example figures, not recommended depths or areas; always use the ones in your own tender documents.

1. Measure the area. Take the net area of the bed from the drawings. Example figure: 250 m2.
2. Find the specified depth. Read it from the detail or specification. Example figure: if the spec calls for 300 mm of planting mix over 250 m2.
3. Convert the depth to metres. 300 mm divided by 1,000 is 0.3 m.
4. Multiply. 250 m2 x 0.3 m = 75 m3 of planting mix in place.
5. Adjust for compaction. Apply the factor from the specification or supplier to get the loose volume to order.
6. Record it as its own line item. Note the product, depth, area and source drawing so the quantity can be checked later.

Where a bed has more than one layer, such as subsoil beneath planting mix, repeat the process for each layer and list each separately.

Why is delivered volume different from in-place volume?

Delivered volume is different from in-place volume because soil and mulch arrive loose and then settle or are compacted once they are placed. A cubic metre on the truck does not fill a cubic metre of bed once it has been spread, watered in and trafficked, so ordering exactly the in-place volume leaves you short.

A compaction factor is the adjustment that converts the compacted, in-place volume you measured into the loose volume you need to order. It varies with the product, its moisture content, how it is placed and how much compaction the specification requires, which is why no single figure suits every job.

Take the compaction factor from the specification if it gives one, otherwise from the supplier for that specific product, or from your own records of similar jobs. Keep the measured in-place quantity and the factor visible as separate figures in your workings, so anyone checking the takeoff can see both.

Also check how the bill of quantities wants the item measured. Some bills ask for the in-place quantity and expect the compaction to be allowed for in the rate, others expect a supplied quantity. Getting that wrong either double-counts the factor or leaves it out.

How do you calculate mulch quantities?

Mulch is calculated the same way as soil: area in square metres multiplied by specified depth in metres gives volume in cubic metres. As a made-up example, if the spec calls for 75 mm of mulch over 400 m2 of beds, that is 400 m2 x 0.075 m = 30 m3 in place.

Mulch also settles after placement, so take the settlement allowance from the specification, supplier or your records. Different mulch products named in the specification, such as a pine bark in one area and a hardwood chip in another, are separate line items even if they share a depth.

How do you calculate turf quantities?

Turf is measured by area, not volume. The quantity in the takeoff is the net area of lawn in square metres, measured from the drawings, and the specification names the turf variety and any soil preparation beneath it.

When ordering, convert the net area into the supplier's roll or slab size, rounding up to whole units. Roll and slab sizes differ between suppliers and varieties, so confirm the size with the supplier rather than assuming one.

Wastage from cuts and edges is real, especially on curved edges and narrow strips. Take your wastage allowance from your own job records for similar layouts, and keep it separate from the measured net area so the bill shows what the drawings actually contain.

How do you measure irregular areas?

Irregular areas are measured by breaking them into simple shapes, working out the area of each and adding them up. Rectangles are length x width, triangles are half of base x height, and circles are pi x radius squared. Curved garden beds can be approximated with several smaller shapes, and the more shapes you use the closer the result.

On commercial jobs with many beds, takeoff software is faster and more accurate: you trace each area on a scaled PDF and the software calculates it, keeping a visual record of what was measured. Either way, check the drawing scale first.

Our guide on how to do a landscape quantity takeoff covers measuring from drawings in more detail, and common landscape takeoff mistakes lists the errors to check for before you submit.

How do you convert cubic metres to tonnes?

To convert cubic metres to tonnes, multiply the volume by the density of the specific product. The density must come from the supplier, because it varies with the mix, the moisture content and how the material is measured. A generic figure for soil can be well out for the product actually specified.

Keep the cubic metre figure as your measured quantity and show the conversion to tonnes as a separate step, with the density and its source noted.

Should each mix and depth be a separate line item?

Yes. Each specified mix, and each different depth of the same mix, should be its own line item. A planting mix at one depth in raised planters and at another in ground-level beds is priced and placed differently, so combining them hides the quantities that your rates depend on.

Separate line items also make the bill easy to check and adjust. A bill of quantities built this way is also the basis for pricing any later contract variation.

Summary table

The table below sets out how each common soft landscape item is measured and what to check before it goes into the bill.

Frequently asked questions

What is the formula for topsoil volume?

Volume in cubic metres equals area in square metres multiplied by depth in metres. Convert the specified depth from millimetres to metres first by dividing by 1,000. That gives the compacted, in-place volume, which you then adjust using the compaction factor from the specification or supplier.

How do you convert millimetres to metres for a soil depth?

Divide the depth in millimetres by 1,000. A depth of 300 mm becomes 0.3 m, 150 mm becomes 0.15 m and 75 mm becomes 0.075 m. Forgetting this step, or dividing by 100 instead, is one of the most common and most expensive errors in a soil takeoff.

Why is the delivered volume of soil more than the in-place volume?

Soil and mulch arrive loose and settle or are compacted once placed, so a cubic metre on the truck fills less than a cubic metre in the bed. Take the compaction factor from the specification or the supplier rather than a rule of thumb, and apply it to the in-place volume.

How do you measure turf for a landscape tender?

Measure turf as a net area in square metres from the drawings, then convert it to the supplier's roll or slab size when ordering. Wastage from cuts and edges is real, especially on curved or narrow areas, so take the allowance from your own job records.

How do you convert cubic metres of soil to tonnes?

Multiply the volume by the density of the specific product, which you get from the supplier. Density varies with the mix and its moisture content, so a generic figure can be well out. If the bill of quantities asks for tonnes, confirm the density before converting.

Should different soil mixes be separate line items?

Yes. Each specified mix, and each different depth of the same mix, should be its own line item. They are priced differently and often supplied differently, so combining them hides the quantities your rates depend on and makes the bill harder to check.

Key takeaways

• Volume (m3) = area (m2) x depth (m); convert millimetres to metres by dividing by 1,000.
• Take every depth from the specification and details, not from habit, and query any conflict.
• Loose delivered volume is more than compacted in-place volume: take the factor from the spec or supplier.
• Turf is measured by net area and ordered in the supplier's roll or slab size, with wastage from your records.
• Convert to tonnes only with the supplier's density for the specific product.
• Give each mix and each depth its own line item so the bill is easy to price and check.