# Concrete Volume & Bag Calculator > Free concrete calculator: work out the volume, bags to buy and the cost for slabs, footings, columns and stairs — multiple elements in one project, metric or imperial, in your browser. Work out the volume, bags to buy and the cost — slabs, footings, columns and stairs, all in one project and entirely in your browser. - **HTML page:** https://siteometry.com/en/tools/concrete - **Category:** [Concrete & Masonry](https://siteometry.com/en/tools/category/concrete-masonry) - **Figures live on the page:** tables, diagrams and worked figures the text refers to ("the table below") are drawn by the page and are not reproduced here. Each section below links to its anchor on the page. - **Runs in the browser:** Siteometry calculators need no account and send no input anywhere — every number below is computed on the visitor's own device. ## Estimate your concrete in six steps 1. **Choose units** — Pick metric (m, mm, kg, m³) or imperial (ft, in, lb, ft³); every value converts when you switch. 2. **Set the bag weight and grade** — Enter the bag weight you'll buy and the concrete grade, which set the default bag yield. 3. **Add an element** — Add an element for each pour and pick its shape — slab, footing, column, stair or custom box. 4. **Enter the dimensions** — Type the labelled dimensions and the quantity; the calculator works out the volume. 5. **Check the waste allowance** — Adjust the waste percentage for spillage and over-excavation; 10% is a safe default. 6. **Read the results, export or print** — Check the volume, bags and cost, then export to CSV or print the shopping list. _On the page: https://siteometry.com/en/tools/concrete#howto_ ## Concrete volume and bag calculator guide The calculator above turns the dimensions of a pour into the answers that matter on site — the volume in cubic metres or yards, the bags of premix to buy, and what it costs — for a single element or a whole project of slabs, footings, columns and stairs, in metric or imperial, all in your browser. The guide below explains how concrete volume is worked out for each shape, how bag counts come from the mix yield rather than guesswork, why a waste allowance protects you against under-ordering, and when a pour is big enough to order ready-mix instead of bags. _On the page: https://siteometry.com/en/tools/concrete#concrete-guide-intro_ ## How much concrete do I need? Concrete is estimated by volume first and bags second. Work out the volume of each element from its dimensions, add them up, add a waste allowance, then divide by how much one bag of premix yields to get the number of bags. In short: volume = the sum of each element's solid volume; bags = volume × (1 + waste) ÷ bag yield, rounded up. The calculator does every step as you type, totals multiple elements into one shopping list, and shows the volume in cubic metres (or cubic yards) so you can switch to ready-mix for a large pour. _On the page: https://siteometry.com/en/tools/concrete#how-much-concrete_ ## How this calculator works You describe the pour; the calculator handles the arithmetic. Add an element, pick its shape from the visual cards, and enter the labelled dimensions — always dimensions, never a raw volume, so a unit mix-up can't silently change your order. Set the bag weight, the concrete grade and a waste allowance once at project level, and they apply to every element. Each element returns its solid volume and bag count, and the project totals the volume, bags and cost across every slab, footing, column and stair. Switch between metric (m, mm, kg, m³) and imperial (ft, in, lb, ft³/yd³) at any time and every value converts. Results can be exported to CSV for a spreadsheet or printed as a clean shopping list. Everything is calculated on your device — nothing you enter is uploaded, and your project is saved only in your own browser. _On the page: https://siteometry.com/en/tools/concrete#how-calculator-works_ ## Volume by shape Every element is a simple solid, and its volume comes straight from the geometry. The calculator uses these formulas, then multiplies by the quantity for repeated elements like a row of identical columns. _On the page: https://siteometry.com/en/tools/concrete#shapes-and-volume_ ## Bags, grade and waste Premix bags are sold by weight, so the bag count depends on how much mixed concrete one bag yields. The calculator derives a default yield from the bag weight and grade — about 0.012 m³ for a 25 kg bag, or 0.6 ft³ for an 80 lb bag — and you can override it with your product's stated figure. A waste allowance protects against under-ordering from spillage, uneven sub-grade and over-excavation; 10% is a sensible default. The grade you choose is recorded with the estimate and nudges the bag yield, because richer mixes pack slightly less volume per bag. _On the page: https://siteometry.com/en/tools/concrete#bags-and-waste_ ## Bags or ready-mix? Bags suit small pours; beyond roughly half a cubic metre, mixing dozens of bags by hand stops being practical and ready-mix delivered by truck is cheaper and more consistent. When a project's total volume crosses that threshold, the calculator flags it and estimates the truck loads, using the cubic-yard or cubic-metre figure that ready-mix is ordered in. Whichever you choose, the volume is the number to quote: order ready-mix by volume, and buy bags by the count the calculator rounds up to. _On the page: https://siteometry.com/en/tools/concrete#ready-mix_ ## Worked examples Each example reproduces what the calculator returns, so you can check the method by hand. Figures use a 25 kg bag at grade C25 with a 10% waste allowance unless stated. _On the page: https://siteometry.com/en/tools/concrete#examples_ ## Common mistakes to avoid Estimating by volume but forgetting waste. A pour that needs 2 m³ exactly will come up short the moment any concrete spills or the sub-grade is uneven — always carry an allowance. Trusting a generic “bags per m³” figure. Bag yield depends on the bag weight, so a 20 kg and a 40 kg bag of the same mix cover very different volumes — set the bag weight you are actually buying. Ignoring the quantity field. Four identical columns are four times the concrete; enter the count rather than adding the element four times. Mixing up depth and thickness units. Slab depth is entered in millimetres or inches, not metres or feet — a slab is 100 mm thick, not 100 m. _On the page: https://siteometry.com/en/tools/concrete#mistakes_ ## Frequently asked questions Short answers to the questions people most often ask about estimating concrete volume, bags and cost with this calculator. ### How much concrete do I need? Work out the volume of each element from its dimensions, add them together, add a waste allowance, then divide by the volume one bag yields to get the bags. For a 5 m × 4 m slab 100 mm thick that's 2 m³, or about 189 × 25 kg bags with 10% waste. The calculator does this as you type and totals multiple elements into one figure. ### How many bags of concrete do I need? Divide the volume (including waste) by how much one bag yields and round up. A 25 kg bag yields roughly 0.012 m³ and an 80 lb bag about 0.6 ft³, so a 2 m³ pour needs around 189 × 25 kg bags. Set the bag weight you're buying and the calculator works out the count. ### How much concrete does one bag make? It depends mainly on the bag's weight: an 80 lb (36 kg) bag yields about 0.6 ft³ (0.017 m³), a 25 kg bag about 0.012 m³ and a 20 kg bag about 0.0094 m³. The grade nudges it slightly. The calculator derives a default from the weight and grade, and you can override it with your product's stated yield. ### How much waste should I allow for concrete? Around 10% is a sensible default to cover spillage, uneven sub-grade and over-excavation. Increase it for rough ground or awkward access and reduce it for a clean, formed pour. The calculator applies the allowance to the volume before working out the bags. ### How do I calculate concrete for a slab? Multiply length × width × depth, all in the same units. A 5 m × 4 m slab 100 mm thick is 5 × 4 × 0.1 = 2 m³. Pick the slab shape, enter the two spans in metres or feet and the depth in millimetres or inches, and the calculator returns the volume and bags. ### How do I calculate concrete for a column? For a rectangular column it's side a × side b × height; for a round column it's π × (diameter ÷ 2)² × height. Enter the quantity for a row of identical columns and the calculator multiplies for you — four 300 mm columns 3 m tall come to about 1.08 m³. ### How do I calculate concrete for stairs? The calculator models a staircase as stacked triangular steps: volume = width × rise × run × N(N+1)/2. Enter the number of steps, the rise and run per step and the width. It excludes any waist slab beneath the steps — add a custom box element if your design has one. ### Should I use bags or ready-mix? Bags suit small pours; beyond about half a cubic metre, ready-mix delivered by truck is usually cheaper and more consistent than mixing dozens of bags. When the project total crosses that threshold the calculator flags it and estimates the truck loads, using the cubic yards or metres ready-mix is ordered in. ### Can I calculate several elements at once? Yes. Add an element for each slab, footing, column or stair and the calculator totals the volume, bags and cost into one project. Project settings — bag weight, grade, waste and price — are entered once and apply to every element. ### Does it work in metric and imperial? Yes. Enter spans in metres or feet, thicknesses in millimetres or inches, and bag weight in kilograms or pounds. Volume shows in cubic metres or cubic feet, with cubic yards for ready-mix. Switch the unit toggle at any time and every value converts. ### Is the calculator private? Yes. Every calculation runs in your browser — nothing you enter is uploaded to a server. Your project is saved only in your own browser's local storage, so you can close the tab and return to it later. _On the page: https://siteometry.com/en/tools/concrete#faq_ ## Related calculators - [Sonotube & Pier Concrete Calculator](https://siteometry.com/en/tools/sonotube-calculator) - [Concrete Mix Ratio Calculator](https://siteometry.com/en/tools/concrete-mix-ratio-calculator) - [Slab Reinforcement Calculator](https://siteometry.com/en/tools/slab-reinforcement-calculator) - [Brick & Block Calculator](https://siteometry.com/en/tools/brick-block-calculator)