Concrete Calculator

Concrete Calculator

How much concrete does the job take: volume in cubic feet, cubic yards and cubic metres, the bags to buy in each common size, and what ready-mix would cost. Six shapes cover nearly every pour, slabs, walls, footings, round tubes and post holes, square columns and steps, and a project list lets you add pour after pour and take off the whole job at once. Bag counts always round up and waste is added openly, because running short mid-pour is the one mistake you cannot fix with a calculator afterwards.

Prices (optional)

Disclaimer: This calculator gives estimates for general information only. It is not financial, medical or other professional advice. It works out volumes and quantities only: slab thickness, reinforcement, mix design and footings that carry loads are engineering and code questions, and local building codes and a qualified professional govern them.

What this calculator does

Concrete is bought by volume and placed by shape, and the gap between the two is where money leaks. This calculator starts from the shape: a slab, a wall, a footing, a round tube or post hole, a square column, or a flight of steps, and returns the volume three ways, cubic feet for bag maths, cubic yards for ready-mix orders, cubic metres for everyone else. It counts the bags in all four common sizes using the yields the manufacturers print on the bags, adds a waste allowance openly rather than burying it, subtracts openings and voids, and with the project list it takes off a whole job pour by pour: slab, footings, columns, steps, each with its own line and a running total. Two honest prices can be attached, per bag or per cubic yard, so the bags-versus-ready-mix decision can be made on numbers.

How to use it

  1. Measure the pour, not the plan. Use the actual formed dimensions: a slab's length and width in feet and its thickness in inches (4 inches is the common residential slab, entered as 4, not 0.33). Post holes and tubes take the diameter across the circle, not the radius, the single most common input error in this class of calculator, and it quadruples the answer when it happens.
  2. Waste is not padding for its own sake. Ten percent is the standard allowance because subgrades are never perfectly level, forms bulge, spills happen and the last wheelbarrow never quite arrives. On a well-prepared flat slab 5% is defensible; on rough excavated footings 10 to 15% is honest. The results show the net volume and the with-waste volume separately so you can see exactly what the allowance costs.
  3. Bags are for small pours; the crossover comes fast. The bag table prices the job all four ways. As a rule of thumb the maths teaches quickly: a cubic yard is roughly 45 bags of 80 lb, so anything past half a yard has you pricing ready-mix as well, where the minimum-load and delivery fees in your area become the real comparison. The ready-mix order quantity rounds up to the quarter yard, the way suppliers sell it.
  4. The project list is the takeoff. Calculate each pour, click "Add this pour to the project list", and the totals rebuild: net volume, with-waste volume, total bags (rounded up once, on the total, the way you actually buy them) and costs if prices were entered. A driveway with footings, a slab and two steps stops being three separate guesses.
  5. Voids subtract before quantity multiplies. An opening in a slab, a sleeved pipe run through a footing: enter its volume per pour in whichever cubic unit you measured it in, and it comes off each pour before the quantity and waste are applied.
  6. Steps are priced as stacked boxes. Each step is a box one riser high and one tread deep, sitting on the ones below; the calculator sums the staircase exactly that way, which is how the formed volume actually works out.

How it is calculated

Volumes: slab, wall and footing = length x width x thickness (or depth). Round column, tube or post hole = pi x (diameter/2) squared x height. Square column = side squared x height. Steps = width x tread x riser x (1 + 2 + ... + number of steps). Net volume = (one shape minus voids) x quantity. Order volume = net x (1 + waste/100). Conversions: 1 cubic yard = 27 cubic feet; 1 cubic metre = 35.3147 cubic feet. Bags = order volume in cubic feet / yield per bag, rounded up, using the standard manufacturer yields: 80 lb bag 0.60 cubic feet, 60 lb 0.45, 50 lb 0.375, 40 lb 0.30. Ready-mix order quantity rounds the with-waste yards up to the next quarter yard.

Worked examples

The patio slab: 10 ft by 12 ft at 4 inches thick. Volume 40.00 cubic feet, 1.48 cubic yards, 1.13 cubic metres. With 10% waste, 44 cubic feet, and the bag table reads 74 bags of 80 lb, 98 of 60 lb, 118 of 50 lb or 147 of 40 lb, which is the bag table's real message: at 74 heavy bags, this slab is a ready-mix conversation.

Four post holes: 12-inch diameter, 4 feet deep, four of them. Each hole is pi x 0.25 x 4 = 3.14 cubic feet; the four take 12.57 cubic feet, 0.47 cubic yards, and 21 bags of 80 lb buy the job with the waste already in. This is the pour size bags were made for.

A flight of steps: 4 feet wide, three steps, 7-inch risers, 11-inch treads. The three boxes stack to 12.83 cubic feet, 0.48 cubic yards: far more concrete than the staircase looks like it holds, which is why steps surprise everyone who guesses.

The metric wall: 5 m long, 2.4 m high, 15 cm thick: 1.80 cubic metres, 63.57 cubic feet, 2.35 cubic yards. The calculator does not care which system you think in; it reports all three volumes every time.

The footing run: 20 feet of footing, 16 inches wide and 8 inches deep: 17.78 cubic feet, 0.66 cubic yards, before waste.

The whole job: add the patio slab and the four post holes to the project list and the takeoff totals 52.57 cubic feet net, 56.57 with the slab's waste, 2.10 cubic yards, 95 bags of 80 lb if bags it must be.

Reading your result

Order on the with-waste figure and plan on the net figure: the difference is your contingency, and on a good day it comes back as a small unopened pile rather than a crisis. For ready-mix, compare the order quantity with the supplier's minimum load and short-load fees before deciding the pour is cheaper by truck; under about a yard, fees often erase the saving, and the bag table is standing right there with the alternative. Check the bag count against your vehicle and your back: 74 bags at 80 lb is 5,920 lb of lifting, a fact the results make it easy to compute and hard to ignore.

Limitations

  • Volume only, not engineering. This page deliberately does not say how thick your slab should be, what strength of mix to order, or how much reinforcement anything needs. Those are code and engineering questions with soil, load and climate inside them, and the disclaimer below is not boilerplate.
  • Yields are the manufacturers' standard figures. Actual yield varies a little with water content and how the mix is placed; the yields printed here are the ones on the bags (0.60, 0.45, 0.375 and 0.30 cubic feet), and the ceiling rounding on bag counts absorbs the small variance.
  • Ground is never the shape you drew. Excavations overbreak, subgrade settles, forms move. The waste allowance is the calculator's answer to that; on hand-dug footings in soft ground, experienced pourers run it above the default, and the net-versus-waste display lets you price the decision.
  • Prices are yours to enter and date. Bag and ready-mix prices are local and move with fuel and cement markets. Nothing is pre-filled, because a plausible-looking wrong price is worse than an empty field.

Frequently asked questions

How many bags of concrete do I need for a slab?

Work out the slab's volume in cubic feet (length x width x thickness, all in feet), add your waste allowance, and divide by the yield of the bag size you are buying: 0.60 cubic feet for an 80 lb bag, 0.45 for 60 lb, 0.375 for 50 lb, 0.30 for 40 lb, rounding up. The worked patio above: 44 cubic feet with waste, so 74 of the 80 lb bags. This page's bag table does that division for all four sizes at once, which matters because the count, and the lifting, change a lot by size.

How much concrete is in a cubic yard, in bags?

A cubic yard is 27 cubic feet, so roughly 45 bags of 80 lb, 60 of 60 lb, 72 of 50 lb or 90 of 40 lb. Those numbers are why the bags-versus-truck decision usually settles itself around half a yard: beyond it, you are mixing for a long time to save money that delivery fees were never going to take anyway.

Why does the calculator add waste instead of just measuring better?

Because the hole is never exactly the drawing. Subgrade varies by fractions of an inch across a slab and fractions of an inch across 120 square feet is cubic feet of concrete; forms bow outward under pressure; some mix stays in the chute, the pump line and the barrow. Running short is the catastrophic outcome: a cold joint in a slab, a second delivery at minimum-load prices, or a footing left low. Ten percent is cheap insurance, and the page shows it as its own line so it is a decision, not a fudge.

Can I use this for a cylinder like a water tank base or a pier?

Yes: the round shape is the cylinder, pi times the radius squared times the height, entered as diameter and height (or depth, for a hole). Piers, sonotube footings, tank pads and post holes are all the same geometry. Enter the diameter across the full circle; halving it first and entering the radius is the one error that quadruples an order.

Related calculators

Method: Euclidean volumes per shape as defined on this page, voids subtracted per pour, quantity and waste applied in that order; bag yields as printed by manufacturers for pre-mixed bags (80 lb 0.60, 60 lb 0.45, 50 lb 0.375, 40 lb 0.30 cubic feet); bag counts rounded up; ready-mix order quantities rounded up to the quarter cubic yard. Last reviewed: 13 October 2026.