How the calculator works out concrete volume
A flight of solid concrete steps is a stepped prism sitting inside a rectangular block whose height equals the total rise and whose depth equals the total going. Half of that rectangular block is solid concrete steps and half is the triangular void above them that never gets filled — assuming your risers and treads are reasonably even. The calculator's core formula is: volume = (total rise x total going x width) / 2, converted to cubic metres or cubic yards, then it adds a waste and compaction allowance of 5-10% because concrete never fills formwork perfectly, air pockets form around rebar, and every wheelbarrow load loses a little on the way from the mixer to the shutter.
Total rise is simply the vertical distance from the lower slab or footing to the top landing. Total going is the horizontal distance the flight covers, front face of the bottom step to the top nosing. Width is the usable tread width, which in the US is governed by IRC R311.7 at a 36in minimum clear width for the main egress stair, and in the UK by Approved Document K, which asks for at least 800mm clear on a private stair with more required on common or public stairs. Whatever width you actually intend to build is the width to enter, not the code minimum, because most homeowners build a little wider than the bare minimum for comfort.
Worked example: a 12-step exterior flight
Take an exterior concrete stair with a total rise of 2.1m (6ft 11in) and a total going of 2.7m (8ft 10in), built 1.2m (3ft 11in) wide. The rectangular envelope is 2.1 x 2.7 x 1.2 = 6.804 cubic metres. Halve that for the stepped shape and you get 3.402 cubic metres of solid concrete before waste. Add 8% for compaction and spillage and you are ordering 3.67 cubic metres, which most UK ready-mix suppliers will round up to 3.75m3 because they deliver in 0.25m3 increments on a mini-mix truck.
In US units the same flight is 6ft 11in rise by 8ft 10in going by 3ft 11in wide. Converting to feet: 6.92 x 8.83 x 3.92 = 239.5 cubic feet for the full envelope, halved to 119.75 cubic feet of stepped concrete, plus 8% waste is about 129.3 cubic feet. Divide by 27 to get cubic yards: 4.79 cubic yards, which a US ready-mix supplier will usually round to 5 yards because most trucks price in whole-yard increments and a partial-load surcharge applies below a full load anyway.
| Quantity | Metric | Imperial |
|---|---|---|
| Total rise | 2.1 m | 6ft 11in |
| Total going | 2.7 m | 8ft 10in |
| Width | 1.2 m | 3ft 11in |
| Raw stepped volume | 3.40 m3 | 120.1 cu ft |
| With 8% waste | 3.67 m3 | 129.3 cu ft |
| Order (rounded) | 3.75 m3 | 5 cu yd |
Know your numbers in 30 seconds
Enter your step count, tread depth and riser height — the stair runner calculator gives you the exact length to order, in metres and feet, with waste included.
Open the stair runner calculatorSizing the formwork
Formwork for concrete steps is built from riser boards (usually 18-25mm exterior-grade plywood or 3/4in form ply) cut to the width of the flight, braced by side stringers cut to the exact stepped profile you want cast — one on each side of the flight, plus a central stringer on anything wider than about 1.5m (5ft) so the riser boards do not bow outward under wet concrete pressure. Each riser board needs a stake or brace behind it, driven into the sub-base, because a fluid concrete pour exerts real hydrostatic pressure on the lowest boards even though the mix looks stiff on the barrow.
Cut the side stringers first from 18mm (3/4in) ply or 38mm (2x) timber, marking out the same rise-and-going steps you calculated for the concrete volume, then screw the riser boards across between the two stringers at each step line. Coat the inside faces with a release agent or shuttering oil so the boards strip cleanly once the concrete has cured, typically after 24-48 hours for the formwork even though full strength takes 28 days.
Rebar: how much and where
Concrete is strong in compression and weak in tension, so a stair flight that will see foot traffic and occasional lateral load needs reinforcement even though it looks like a solid mass. The standard approach is a single layer of 10-12mm (US #3-#4) rebar on a grid of roughly 300mm (12in) centres, running both up the slope of the underside slab and across the width, tied into starter bars that key into the existing foundation or slab so the whole flight cannot shear away from the structure it sits against.
- Sloped base slab: one mat of 10-12mm bar at 300mm (12in) centres, both directions.
- Starter bars: 300-400mm (12-16in) long, doweled into the adjoining slab or footing with epoxy anchor or cast in with the footing pour.
- Cover: minimum 40mm (1.5in) of concrete cover to any exterior-exposed rebar, more in freeze-thaw climates.
- Mesh option: A193/A252 mesh (UK) or 6x6 W1.4/W1.4 welded wire mesh (US) is an acceptable lighter-duty alternative on small domestic flights under three steps.
Do not skip the sub-base
Concrete steps poured directly onto disturbed or poorly compacted ground will crack and settle within a couple of seasons. Compact a 100-150mm (4-6in) crushed stone sub-base under the whole footprint of the flight before you build formwork, and check local frost depth rules — in cold-climate US states the footing typically needs to sit below the frost line, commonly 900mm-1.2m (36-48in).
Cost per unit volume: UK and US
Ready-mix concrete delivered to site in the UK typically runs £110-£160 per cubic metre for a standard C25/C30 mix, with a small-load surcharge below about 4m3 that can add £50-£100 to the ticket. In the US, ready-mix concrete typically runs $130-$180 per cubic yard for a comparable 3000-4000 psi mix, again with a short-load fee below a full truck (usually 8-10 cubic yards) that can add $75-$150. On top of the raw material, budget for formwork lumber (often reusable across two or three jobs), rebar, a release agent, and either a mixer hire or hand-mixing for anything under about 1m3 / 1.3yd3, below which many suppliers will not deliver at all.
| Cost item | UK typical | US typical |
|---|---|---|
| Ready-mix per unit | £110-£160 / m3 | $130-$180 / cu yd |
| Short-load surcharge | £50-£100 under 4m3 | $75-$150 under 8-10 cu yd |
| Formwork lumber (reusable) | £60-£150 | $80-$180 |
| Rebar for a 12-step flight | £70-£120 | $90-$150 |
| Full DIY exterior flight (materials only) | £350-£600 | $450-$750 |
| Contractor-built flight, labour + materials | £1,200-£2,800 | $1,600-$3,600 |
Riser and going proportions still apply
Even though the material is concrete rather than timber, the same comfort rule that governs any staircase still applies: 2 x riser + 1 x going should land between roughly 600-650mm (24-25.5in). Exterior concrete steps are typically built with a slightly lower riser than an interior stair — 150-175mm (6-7in) rather than 175-200mm (7-8in) — because outdoor steps are used in wet and low-light conditions and a shallower riser is safer underfoot. UK Building Regs Part K sets a maximum going rise ratio and a minimum going for external steps that mirrors the internal stair rules, while US IRC R311.7 caps riser height at 7.75in (196mm) and requires a minimum 10in (254mm) going for the main flight.
- Confirm the finished floor level at the top and the ground or slab level at the bottom to fix total rise.
- Choose a riser height in the 150-175mm (6-7in) range and divide total rise by it to get the number of steps, rounding to a whole number.
- Recalculate the exact riser height by dividing total rise by the rounded step count so every riser is identical.
- Set the going using the 2R+G comfort rule, then multiply by the step count to check the total going fits the available run.
- Feed the confirmed rise, going and width into the concrete stair calculator to get your final order volume.
Finishing and curing
Concrete steps need broom-finishing or a light exposed-aggregate texture on the tread surface for slip resistance while the surface is still workable, roughly 30-90 minutes after the pour depending on temperature. Cover the finished flight with plastic sheeting or a curing compound for the first 3-7 days to stop the surface drying out and cracking, and keep foot traffic off entirely for at least 24-48 hours and vehicle or heavy loading off for the full 28-day cure. In cold weather, an insulating blanket over freshly poured exterior steps prevents the surface from freezing before it has gained enough strength.
Whatever you decide, get the numbers first: the stair runner calculator doubles as a stair runner length calculator, adding nosing wrap, landings, winders, pattern repeat and waste so you order the right length the first time.
Frequently asked questions
How much concrete do I need for stairs?
Multiply total rise by total going by width, divide by two for the stepped shape, then add 5-10% waste. A typical 12-step exterior flight 1.2m wide needs roughly 3.5-3.75 cubic metres (about 4.8-5 cubic yards) once waste is included. Always round up to your supplier's delivery increment rather than ordering the bare minimum.
What size rebar do concrete steps need?
Most domestic flights use 10-12mm bar (US #3-#4) laid in a grid at roughly 300mm (12in) centres in both directions, tied into starter bars doweled into the adjoining slab or footing. Small flights of two or three steps can often use welded wire mesh instead of individual bars.
How thick should concrete steps be?
The solid section under each tread should be at least 150mm (6in) thick measured perpendicular to the slope, and the base slab beneath the whole flight should sit on a compacted 100-150mm (4-6in) sub-base. Thinner sections are prone to cracking under repeated foot traffic and freeze-thaw cycling.
How much does a concrete stair calculator save compared to guessing?
Ordering by guesswork typically leads to either an expensive short-load surcharge from under-ordering or 10-20% of wasted concrete from over-ordering. A calculator that works from actual rise, going and width numbers usually gets the order within one delivery increment of what you actually need.
Do concrete steps need expansion joints?
Yes, where the stair meets an existing slab or foundation, a flexible joint filler prevents differential movement from cracking either structure. This matters most in climates with significant freeze-thaw cycling or expansive clay soils.
Can I use a stair runner length calculator on concrete steps?
The stair runner length calculator is built for finished timber flights with a runner and gripper, but the same rise-and-going geometry it uses is exactly what feeds the concrete stair calculator's volume formula, so working out your going and rise there first is a useful cross-check before you order concrete.
How long before concrete steps can be walked on?
Light foot traffic is usually safe after 24-48 hours, but full strength and resistance to heavy use or de-icing salt takes the standard 28-day cure. In cold weather, extend the no-traffic period by a further day or two and protect the surface with an insulating cover.
Know your numbers in 30 seconds
Enter your step count, tread depth and riser height — the stair runner calculator gives you the exact length to order, in metres and feet, with waste included.
Plan my stair geometryEvery guide is written from hands-on measuring and fitting experience, checked against the calculator’s own maths, and re-reviewed whenever prices or methods change. Read our editorial standards.