Concrete Calculator
This concrete calculator estimates how much concrete a rectangular slab needs in cubic feet and cubic yards. Enter the slab length in feet, width in feet, thickness in inches, and an optional waste allowance to get both the base volume and the recommended order quantity. It works well for patios, shed pads, sidewalks, landings, garage floors, and other flat pours where the shape is close to a rectangle. Concrete is usually purchased either as bagged mix or by the cubic yard from a ready-mix supplier. Small jobs may be practical with bags, while larger pours are usually easier to order by the yard. This page helps you do the volume math first so you can compare bag counts, truck quotes, and waste allowance before you buy materials.
Quick answer
The calculator converts slab thickness from inches to feet before it multiplies length, width, and thickness to get cubic feet.
What this tells you
- •The calculator converts slab thickness from inches to feet before it multiplies length, width, and thickness to get cubic feet.
- •It then divides cubic feet by 27 because 1 cubic yard equals 27 cubic feet, which is the unit most ready-mix suppliers quote.
- •Waste percentage increases the final yardage so your order has room for spillage, uneven grade, form leakage, and minor overexcavation.
- •Bag counts in the reference table are planning shortcuts based on typical yields for 40 lb, 50 lb, 60 lb, and 80 lb ready-mix bags.
- •The result is strongest for simple rectangular slabs. Curved edges, thickened edges, footings, and round piers need separate volume steps.
How to Use
- 1Enter the slab length in feet. Measure the full pour from one form edge to the opposite edge, not just the finished walking area.
- 2Enter the slab width in feet. If the project has more than one rectangle, calculate each section separately and add the yardage after that.
- 3Enter thickness in inches. A 4 inch slab should be entered as 4, not 0.333, because the formula converts inches to feet for you.
- 4Add a waste allowance if you want a safer order number. Many homeowners use 5% to 10%, while rougher sites or first-time pours may need more.
- 5Review the cubic feet, cubic yards, and total with waste. Then compare the yard figure with the bag table to decide between mixing bags or ordering ready-mix.
How It Works
Formula
Thickness (ft) = Thickness (in) / 12
Cubic Feet = Length(ft) x Width(ft) x Thickness(ft)
Cubic Yards = Cubic Feet / 27
Total with Waste = Cubic Yards x (1 + Waste% / 100)The calculator follows the same sequence a contractor would use on paper. It first changes slab thickness from inches to feet because length and width are already measured in feet. After that, it multiplies length by width by thickness to get the slab volume in cubic feet. Since concrete suppliers usually sell ready-mix in cubic yards, the tool divides by 27 to convert cubic feet into cubic yards. If you add a waste percentage, the calculator multiplies the yardage by 1 plus the waste rate expressed as a decimal. For example, a 10% waste allowance uses a multiplier of 1.10. The final result is a planning estimate for ordering concrete, not a structural design check.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
Patio slab estimate
Thickness in feet is 4 / 12, which is about 0.3333. Multiply 20 x 10 x (4 / 12) to get 66.67 cubic feet, then divide by 27 to get 2.47 cubic yards. Adding 10% waste gives 2.47 x 1.10 = 2.72 cubic yards. That is a realistic patio size where bagged concrete starts to become a lot of labor.
10 x 10 shed pad
A 10 x 10 slab at 4 inches thick contains 10 x 10 x (4 / 12) = 33.33 cubic feet of concrete. Dividing by 27 gives 1.23 cubic yards before waste. With a 5% allowance, the order number becomes 1.23 x 1.05 = 1.30 cubic yards. That is a helpful benchmark when comparing a small ready-mix delivery fee against about 56 bags of 80 lb mix.
Two-car garage slab
Convert thickness first, so 5 inches becomes 5 / 12 feet. The slab volume is 24 x 24 x (5 / 12) = 240.00 cubic feet, which equals 240 / 27 = 8.89 cubic yards. After an 8% waste allowance, the recommendation is 8.89 x 1.08 = 9.60 cubic yards. At this size, ready-mix delivery is almost always more practical than mixing bags on site.
Straight sidewalk pour
This sidewalk uses a common 4 inch thickness, which is 4 / 12 feet. Multiply 30 x 4 x (4 / 12) to get 40.00 cubic feet, then divide by 27 for 1.48 cubic yards. Adding 7% waste gives 1.48 x 1.07 = 1.59 cubic yards. The example shows how even a narrow walk can move past one full yard quickly when the run is long.
Hot tub pad
A 6 inch slab is 0.5 feet thick, so the raw volume is 8 x 8 x 0.5 = 32.00 cubic feet. Dividing by 27 gives 1.19 cubic yards before waste. With a 10% buffer, the total becomes 1.19 x 1.10 = 1.30 cubic yards. Thicker pads for heavier loads can use more concrete than their footprint suggests, which is why thickness deserves a careful check.
Concrete Bags Per Cubic Yard
Use this table to compare a cubic-yard order with the number of ready-mix bags you would need to mix by hand. Actual yield can vary slightly by product and moisture conditions, but these figures are common planning estimates.
| Bag size | Typical yield per bag | Bags for 1 cubic yard | Common use |
|---|---|---|---|
| 40 lb | 0.30 cubic feet | 90 bags | Small patches, anchors, and minor repairs |
| 50 lb | 0.375 cubic feet | 72 bags | Short runs where lighter bags are easier to handle |
| 60 lb | 0.45 cubic feet | 60 bags | Medium jobs with a balance of weight and yield |
| 80 lb | 0.60 cubic feet | 45 bags | Slabs and pads where fewer bags save mixing time |
Check the yield printed on the bag you plan to buy. Fast-setting, high-strength, and specialty mixes can produce a slightly different volume per bag.
When bags stop making sense
Bagged concrete is convenient because you can buy only what you need, transport it in a pickup, and mix small batches without scheduling a truck. That works well for post holes, stair landings, tiny pads, or repair work where the total volume stays low. It also gives you more flexibility if the forms, base, or weather are not ready for a full delivery window.
The tradeoff is labor. An 80 lb bag yields only about 0.60 cubic feet, so a full cubic yard takes about 45 bags. A patio that needs 2.72 cubic yards is roughly 122 bags of 80 lb mix if you tried to do the same job by hand. At that point, time, fatigue, water control, and batch consistency become real issues.
Ready-mix usually becomes the easier option once a project gets near or above 1 cubic yard. Suppliers can place one consistent mix into the forms faster than a small crew can open, mix, and dump dozens of bags. You still need to check minimum order fees, chute access, wheelbarrow distance, and placement speed, but the yard calculation gives you the number you need for those conversations.
Use the calculator result as the starting point, then match it to the job conditions. If the site is hard to reach, bags may still win for a small pour. If the number is several cubic yards, delivery is often the cleaner path. The right choice depends on both the volume and how hard it is to place the concrete once it arrives.
Common mistakes
- Entering thickness as feet instead of inches. Typing 0.333 into a field that expects inches cuts the slab depth to a fraction of an inch and destroys the estimate.
- Measuring only the finished surface area and forgetting form overhangs, thickened edges, or separate rectangular sections that also need concrete.
- Skipping waste on uneven sites. Sloped subgrade, form leaks, and cleanup loss can leave a carefully measured order a little short.
- Using the yard figure as a bag count without checking bag yield. Bagged concrete is sold by weight, but the planning question is cubic feet produced per bag.
- Assuming volume alone makes the slab buildable. Thickness, reinforcement, base prep, joints, and local code requirements still matter for the actual project.
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