How much concrete to order, and a free calculator to check it

Order too little concrete and the pour stalls while you wait for more. Order too much and you’ve paid for a load that goes back to the plant. The maths is simple, and the allowances around it are where jobs go wrong.

The base sum is length × width × depth, with everything in metres. A 6 m by 4 m slab at 100 mm thick is 6 × 4 × 0.1, or 2.4 m³. Round pours and post holes use π × radius² × depth, so eight 300 mm holes dug 600 mm deep come to about 0.34 m³.

A free calculator that does the sums

A free concrete calculator from TradeQuote, which makes quoting, job and invoicing software for builders, runs those formulas for slabs, strip footings, round pours and post holes. It needs no login, and builders, landscapers and suppliers can embed it on their own websites for free.

You pick a waste allowance of 0%, 5%, 10% or 15%, and choose whether to round the order up to the nearest 0.01, 0.05 or 0.1 m³. It returns the net volume, the volume with waste, an order quantity, an approximate wet weight and bag counts for 20 kg, 25 kg and 30 kg premix. On its defaults of 10% waste and 0.05 m³ rounding, that 2.4 m³ slab becomes an order of 2.65 m³.

Wastage, bags and trucks

How much to add depends on the ground. Allied Concrete, which runs ready-mix plants around the country, recommends a 5% buffer to cover spillage, over-excavation and uneven surfaces. A soft or rough subgrade, or holes dug by hand, can justify more.

The calculator assumes about 9 litres of concrete per 20 kg bag and says to go by the printed yield. Dricon, Firth’s bagged-premix brand, lists 0.010 m³ per 25 kg bag of HandiCrete and 0.016 m³ per 40 kg bag. For the slab plus 10%, that’s 264 of the 25 kg bags, against the calculator’s 235.

Numbers like that make the case for ready-mix on a slab. Bags suit post holes and small pads, and the eight fence posts with 10% extra take 24 of the 40 kg bags.

Before booking a truck, ask the plant about minimum loads and small-load charges, and have the site ready. Allied’s terms of trade let it charge waiting time once a truck has spent more than 10 minutes per cubic metre on site.

Consent and who can pour it

Schedule 1 of the Building Act 2004 exempts some common concrete jobs from building consent. These include fences up to 2.5 m high, other than pool fences, retaining walls holding back no more than 1.5 m of ground with no extra load such as vehicles, and decks you can’t fall more than 1.5 m from. Section 17 of the Act says that work must still comply with the Building Code.

Foundations for a house are restricted building work, so a licensed building practitioner must do or supervise them. That includes small stand-alone dwellings of up to 70 m² built under the consent exemption that took effect on 15 January 2026.

The calculator is at tradequote.co.nz/tools/concrete-calculator.