Estimating concrete takes five steps: measure the space, convert everything to feet, apply the right formula for the shape, convert to the unit you are buying, and add a margin for waste. This guide walks through each step with the math shown, so you can check any number the concrete calculator gives you, or work it out on paper.
Step 1: Measure the space
Accurate measurements matter more than any formula. A small error in thickness spreads across the whole area of a slab.
Length and width
Measure the inside of the forms, not the outside of the boards. For a patio or walkway, measure the longest and widest points. If the shape is irregular, split it into rectangles (see Step 3).
Thickness and depth
Measure the depth in several places, at least at each corner and in the middle, and use the average. Ground that looks flat often isn't.
Take a 12 × 12 ft patio planned at 4 inches. If the excavation averages 4.5 inches, you need 12.5% more concrete than planned, more than a typical waste allowance.
Holes and tubes
For post holes and sonotubes, measure the diameter across the widest point, and the depth from the bottom of the hole to where the concrete will stop. Hand-dug holes are rarely straight cylinders. They tend to bell out at the top, so measure the top diameter and allow extra.
Step 2: Convert everything to feet
Concrete is ordered in cubic yards, so every dimension must be in the same unit before you multiply. Length and width are usually in feet already. Thickness is usually in inches.
| Inches | Feet |
|---|---|
| 2 in | 0.167 ft |
| 3 in | 0.25 ft |
| 4 in | 0.333 ft |
| 5 in | 0.417 ft |
| 6 in | 0.5 ft |
| 8 in | 0.667 ft |
| 12 in | 1 ft |
Divide inches by 12. For mixed measurements such as 12 ft 6 in, convert the inches and add them: 12 + 6 ÷ 12 = 12.5 ft.
For metric, NIST defines the inch as exactly 25.4 mm and the foot as exactly 0.3048 m.
Step 3: Use the formula for your shape
Slabs, patios, walkways and driveways
Volume (ft³) = length × width × thickness (ft)
A 24 × 12 ft patio at 4 inches is 24 × 12 × 0.333 = 96 ft³.
Footings
A footing is a long, narrow slab, measured in inches across and down.
Volume = length (ft) × width (ft) × depth (ft)
A 20 ft footing that is 16 in wide and 8 in deep: 20 × 1.333 × 0.667 = 17.78 ft³.
Walls
Volume = length × height × thickness
A 20 ft wall, 4 ft high and 8 in thick: 20 × 4 × 0.667 = 53.33 ft³.
Round columns and sonotubes
Volume = π × radius² × height, where the radius is half the diameter, in feet.
A 12 in tube has a radius of 0.5 ft. At 4 ft tall: 3.1416 × 0.25 × 4 = 3.14 ft³ per tube.
Post holes
When a post sits in the hole, it takes up space that concrete doesn't fill. Subtract it:
Volume = (π × hole radius² − post width²) × depth
Use the post's actual size: a 4×4 is 3.5 in square and a 6×6 is 5.5 in square. A 10 in hole, 30 in deep, around a 4×4 post is (3.1416 × 0.4167² − 0.2917²) × 2.5 = 1.15 ft³.
Solid steps
Each step is a block resting on the ground, and each one is taller than the last. For n steps:
Volume = width × run × rise × n(n + 1) ÷ 2
Three steps, 4 ft wide, 11 in run, 7 in rise: 4 × 0.917 × 0.583 × 6 = 12.83 ft³.
Irregular and combined shapes
Break the area into rectangles, triangles and circles, work out each one, and add them. For an L-shaped patio, two rectangles that don't overlap are enough. A triangle is ½ × base × height. The square footage calculator can give you the area, and you then multiply by the thickness in feet.
Step 4: Convert to cubic yards, cubic meters or bags
Cubic yards
Cubic yards = cubic feet ÷ 27
The 96 ft³ patio is 96 ÷ 27 = 3.56 yd³. Ready-mix suppliers in the US quote and deliver by the cubic yard.
Cubic meters
Cubic meters = cubic feet × 0.0283168, so 1 yd³ = 0.7646 m³.
Bags of premixed concrete
QUIKRETE's Concrete Mix data sheet lists these yields per bag, and SAKRETE's High-Strength Concrete Mix data sheet matches them for the three common sizes:
| Bag | Yield | Bags per yd³ |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.60 ft³ | 45 |
Bags = cubic feet ÷ yield, rounded up. QUIKRETE also lists 50 lb (0.375 ft³) and 90 lb (0.675 ft³) bags. Fast-setting and specialty mixes can differ, so read the bag.
Step 5: Add a waste allowance
No pour uses exactly the calculated volume. Concrete gets spilled, forms bow slightly, the subgrade settles under the weight, and some is left in the mixer or wheelbarrow. The calculator adds 10% by default.
- Use more (15% or above) for hand-dug holes, uneven or soft ground, or footings dug without forms.
- Use less only for small, formed pours on a compacted, level base.
Running short is the expensive mistake. A second batch mixed hours later can leave a cold joint, a visible seam that may be weaker than continuous concrete. QUIKRETE's data sheet warns against partial-depth fills for this reason.
Ordering: bags or ready-mix?
Weight decides this as much as cost. One cubic yard is 45 × 80 lb bags: 3,600 lb to carry and mix, plus water.
| Job size | Usual approach |
|---|---|
| A few posts, a small pad, under ~0.5 yd³ | Bags, mixed in a wheelbarrow or tub |
| ~0.5 to 1 yd³ | Bags with a rented mixer, or a small-load supplier |
| Over ~1 yd³ | Ready-mix truck |
These breakpoints are rules of thumb, not prices. Ready-mix suppliers often have a minimum load and may charge a fee for small loads. Call two local suppliers with your cubic yard figure and compare the delivered total against the bag count × bag price.
When you order ready-mix, tell the supplier:
- the volume in cubic yards, including waste;
- what you're pouring (slab, footing, wall), so they can suggest a mix;
- how the truck will reach the pour, and whether you need a pump or a wheelbarrow crew.
Mixing bagged concrete correctly
QUIKRETE's data sheet calls for about 6 pints of water per 80 lb bag to start. The final water content should be about 6 to 9 pints, added gradually until the mix is stiff and moldable, with no dry pockets and no standing puddles.
- Mixer size: allow at least 1 ft³ of mixer capacity for each 80 lb bag mixed at one time.
- Don't add too much water. Extra water makes concrete easier to place but weaker.
- Mix fully: check for dry pockets at the bottom of the tub.
Curing: the step most people skip
Curing keeps moisture in the concrete so the cement keeps hydrating and gaining strength. The American Cement Association (formerly the Portland Cement Association) says to start curing once the surface has hardened enough to resist marring. You can cure by:
- sprinkling, fogging, or covering with wet burlap or cotton mats;
- sealing in moisture with plastic sheeting or a curing compound.
QUIKRETE's data sheet says to continue curing for 5 days in warm weather (70 °F or higher) or 7 days in colder weather (50–70 °F). The same sheet lists compressive strength of about 2,500 psi at 7 days and 4,000 psi at 28 days for its Concrete Mix. Concrete keeps gaining strength well after it looks dry.
Common mistakes
- Mixing units. Multiplying feet by inches gives a number 12 times too large.
- Using nominal lumber sizes. Form boards and posts are smaller than their names.
- Measuring thickness once. Average several readings.
- Forgetting the post. Without subtracting it, a post hole estimate runs high. That's harmless, but it overstates cost.
- No waste allowance. Plan for leftovers, not a shortfall.
- Pouring in the wrong weather. Very hot, dry, windy or near-freezing conditions all make curing harder. Check the forecast.
Frequently asked questions
How do I calculate cubic yards of concrete?
Multiply length × width × thickness, all in feet, then divide by 27. A 10 × 10 ft slab at 4 in is 33.33 ft³ ÷ 27 = 1.23 yd³.
How many bags of concrete are in a yard?
45 × 80 lb, 60 × 60 lb or 90 × 40 lb bags, based on the standard yields of 0.60, 0.45 and 0.30 ft³.
Do I need to subtract rebar or mesh?
No. Reinforcement takes up a negligible share of the volume.
How much extra concrete should I order?
10% is a sensible default. Use more for irregular or hand-dug work.
Can I pour concrete in layers on different days?
Not for a slab or a structural element. A later pour bonds poorly to hardened concrete and leaves a cold joint. Order or mix enough to finish in one go.
Sources
- QUIKRETE Concrete Mix (No. 1101) data sheet: yields for 40–90 lb bags, mixing water, curing duration and strength. https://www.quikrete.com/pdfs/data_sheet-concrete%20mix%201101.pdf
- SAKRETE High-Strength Concrete Mix technical data sheet: 40/60/80 lb yields. https://www.sakrete.com/wp-content/uploads/2024/01/Sakrete-High-Strength-Concrete-Mix-TDS.pdf
- American Cement Association (formerly PCA), "Applications of Cement": finishing and curing methods. https://www.cement.org/cement-concrete/applications-of-cement/
- NIST, SI units of length: 1 in = 25.4 mm exactly; 1 yd = 0.9144 m. https://www.nist.gov/pml/owm/si-units-length