How do I work out how much to buy for a DIY building job?
Published · By Sai Nath
Most DIY building jobs come down to one sum: length × width × depth, in metres, for cubic metres, then the supplier's density for tonnes or the bag's printed yield for bags. A 1.8 × 0.9 m raised bed filled 300 mm deep holds 0.486 m³ of soil. Add about 10%, measure depths after compacting, and give paving a fall.
Which calculations does a DIY job need?
Fewer than you'd think. Garden beds, paths, pads and post holes are all volumes. Fences are equal spacing. Steps are one division. This table matches each job to its sum and to the calculator that does it.
| Job | What to work out | In MeisterCalc |
|---|---|---|
| Raised garden bed | cubic metres (or litres) of soil | Soil |
| Gravel path | cubic metres, then tonnes | Gravel |
| Small slab or post holes | cubic metres, then bags | Concrete (free) |
| Fence posts or pickets | equal gaps along a run | Equal Spacings |
| Two or three garden steps | number and height of the steps | Stairs (free) |
| A square corner for a shed base | the diagonal | Check Square (free) |
| A patio that drains | the fall over its length | Pitch |
Measure everything in one unit before you start. The calculators take millimetres (or feet and inches, if you switch them), and most mistakes on small jobs are unit mix-ups.
How much soil do I need for a raised garden bed?
Length × width × depth, in metres. A bed 1800 × 900 mm, filled 300 mm deep, takes 1.8 × 0.9 × 0.3 = 0.486 m³. A cubic metre is 1,000 litres, so that is 486 litres.
- Bagged: soil and mix are sold by the litre. At an example 25 litres a bag (read yours), 486 ÷ 25 = 19.4, so 20 bags, or 22 with about 10% for settling (534.6 ÷ 25 = 21.4).
- Bulk: sold by the cubic metre or the tonne. At the Soil calculator's starting density of 1,200 kg/m³, 0.486 m³ weighs 0.583 t, and 0.642 t with 10%. Use your supplier's figure if it differs.
In the app, Soil with Length 1800 mm, Width 900 mm, Depth 300 mm and Density 1200 kg/m³ shows Volume 0.486 m³, Weight 0.583 t and Order (+10%) 0.642 t. What to fill a bed with, and top dressing a lawn, are in how much soil do I need.
How much gravel do I need for a path?
Cubic metres × density ÷ 1,000 gives tonnes. An 8 m path, 900 mm wide and 75 mm deep after compacting, is 8 × 0.9 × 0.075 = 0.540 m³. At 1,500 kg/m³ that is 0.540 × 1,500 ÷ 1,000 = 0.810 t, so order about 0.89 t. Gravel in MeisterCalc starts at 1,500 kg/m³ (its figure for 20 mm crushed rock); type your supplier's density over it.
If it comes in bulk bags, divide the tonnes by the bag weight on the listing (how many bulk bags of gravel). If you're carting it yourself, a trailer's weight limit usually runs out before it is full: see how many cubic metres a trailer holds.
How much concrete for a small pad or post holes, and how many bags?
Work out the volume, add 10%, then divide by what one bag makes. That last figure is printed on the bag and changes with the brand and bag size.
- A 1 m square pad for a barbecue or bins, 100 mm thick: 1.0 × 1.0 × 0.1 = 0.100 m³, and 0.110 m³ with 10%. Boral's data sheet says 108 of its 20 kg Concrete Mix bags make 1 m³, so 0.110 × 108 = 11.9: buy 12 bags. In the US, 80 lb bags of QUIKRETE Concrete Mix yield 0.60 ft³ each by its data sheet: 3.88 ft³ ÷ 0.60 = 6.5, so 7 bags.
- Five fence post holes, 250 mm (9 13/16″) across and 600 mm (23 5/8″) deep: one hole is π × 0.125² × 0.6 = 0.0295 m³, five are 0.1473 m³, and 0.1620 m³ with 10%. That is 18 bags of the Boral mix, or 10 bags of 80 lb.
The hole sizes are examples; your fence or post maker gives the real ones. Don't subtract the post: hand-dug holes come out bigger than the sum (how much concrete for a post hole).
How do I space fence posts evenly?
Divide the run by the longest bay your panels or rails allow, round up to whole bays, and divide the run by that number. A 9.5 m fence with bays no longer than 2.4 m: 9500 ÷ 2400 = 3.96, so 4 bays of 9500 ÷ 4 = 2375 mm, and 5 posts, one more than the bays. Equal Spacings, Overall Length 9500 mm and Target Spacing 2400 mm, shows Number of Spaces 4, Actual Spacing 2375.0 mm and Pieces Needed 5.
Check the Actual Spacing every time. Equal Spacings rounds to the nearest whole number of spaces, so it can come out wider than you asked: 8000 mm at 2400 gives 3 spaces of 2666.7 mm, too wide for 2.4 m rails. Round up by hand instead (8000 ÷ 2400 = 3.33, so 4 bays) and enter 2000 mm: 4 spaces, 5 posts. Pickets work the same way. Panels and the leftover bay are in how many fence posts do I need.
How many steps do I need up a garden bank?
Divide the height by a comfortable step and round. For a 450 mm bank with 300 mm treads, a comfortable step is (600 − 300) ÷ 2 = 150 mm, and 450 ÷ 150 = 3, so 3 steps up of 150 mm and 2 treads. The treads take 2 × 300 = 600 mm of ground, and 2 × 150 + 300 = 600 mm sits inside the 550 to 700 mm comfort range for 2R + G (the 2R + G formula). In the free Stairs calculator, Rise Only with Total Rise 450 mm and Going 300 mm shows 3 risers of 150.0 mm and 2 treads; its Total Run, 900 mm, counts one more going, onto the top level. For a full flight, use the stair calculator.
What about a DIY deck?
It's the same sums on a bigger frame: square it by the diagonals, footings as post holes, joists, and boards with the gap. A 4.8 × 3.6 m deck needs 6000 mm on both diagonals. The whole deck is worked through, step by step, in the deck building calculator.
What mistakes do DIY builders make with the numbers?
- Buying by the bag without the yield. The pad's 0.110 m³ is 12 bags of Boral Concrete Mix 20 kg, but 14 of Cement Australia's standard 20 kg Concrete Mix, which works out at about 119 bags per m³ from its product sheet. Same weight of bag, two more to buy. The figures for other bags are in bags of concrete per m³.
- Forgetting compaction. Measure the depth after compacting, and ask for the compacted density. If the supplier's gravel is 2,000 kg/m³ compacted (an example figure), the path needs 1.080 t, not the 0.810 t the starting 1,500 gives: 0.27 t short.
- Mixing units. A path measured as 26 ft and typed into a metric box as 26 m orders 3.3 times too much: 26 ft is 7924.8 mm. Convert first.
- No fall on paving. A patio laid dead level holds water. At 1 in 60 a 3 m patio drops 50 mm; next to an Australian slab-on-ground house, NCC 3.3.3 asks for at least 25 mm (low rainfall areas, concrete or clay paving) or 50 mm over the first metre. See how much fall a patio needs.
When should I call a builder, or ask the council?
Before you build anything that holds people up, holds soil back or fixes to the house, and before you dig. A calculator gives quantities; it doesn't size a footing, a joist, a retaining wall or a balustrade. Those come from plans, span tables or an engineer.
- Approval depends on where you live, and this page gives no thresholds for Australia, New Zealand or the US: ask your council or building department before you start. In England, the Planning Portal says decking usually needs no planning application if it is no more than 30 cm above the ground, covers no more than half the garden with other extensions and outbuildings, and isn't in front of the house's principal elevation. That's planning only; Building Regulations are separate.
- Barriers on raised decks and steps follow the building rules: in Australia where a fall of 1 m or more is possible (NCC 11.3), and under the US IRC more than 30 in above the floor or ground below (2018 IRC R312.1, quoted in the Knox County guide; states and cities adopt it with their own changes).
- Before you dig post holes, ask where the buried cables and pipes are. These services are free: Before You Dig Australia, beforeUdig in New Zealand and 811 in the US.
Which of these calculators are free?
For garden jobs you'll use Soil, Gravel, Concrete, Equal Spacings, Stairs, Check Square and Pitch, plus Decking for a deck. MeisterCalc has 16 calculators. Concrete, Stairs and Check Square are free; the other 13 unlock with a weekly subscription (3-day free trial) or a one-time lifetime unlock. Every one draws a live diagram of what you typed, and calculations run on the phone, on Android 7.0 or later.