Ordering the wrong amount of concrete is a costly mistake in either direction. Too little and your pour comes up short, leaving you scrambling to mix emergency bags before the first batch starts setting. Too much and you are paying for a material you cannot return or easily dispose of. Unlike paint or lumber, concrete is unforgiving: once you start pouring, the clock is ticking. The good news is that calculating the right amount is straightforward math, and this guide walks through every formula, conversion, and project type so you get it right before the truck arrives or you load up on bags.
The Core Formula: Length x Width x Depth
Every concrete calculation starts with the same basic concept: you are filling a three-dimensional space, so you need to calculate volume. For rectangular pours like slabs, sidewalks, and patios, the formula is simple:
Concrete Volume Formula
Volume = Length x Width x Depth
Measure all three dimensions in the same unit (feet is easiest), then convert the result from cubic feet to cubic yards by dividing by 27.
Cubic Yards = (Length ft x Width ft x Depth ft) / 27
The depth is the dimension most people get wrong. A 4-inch thick slab is not 4 feet; it is 0.33 feet (4 divided by 12). A 6-inch sidewalk is 0.5 feet. Always convert inches to feet before multiplying, or your volume will be wildly off. If converting between units feels tedious, the square footage calculator handles area math quickly so you can focus on the depth conversion.
Converting to Cubic Yards
Concrete is sold and delivered in cubic yards. One cubic yard equals 27 cubic feet, which is a 3-foot by 3-foot by 3-foot cube. Every concrete calculation ultimately needs to land on a cubic yard number, whether you are ordering a ready-mix truck or buying bags at the hardware store.
Quick Conversion Reference
1 cubic yard = 27 cubic feet
1 cubic yard = 46,656 cubic inches
1 cubic yard = approximately 2 tons of mixed concrete
1 cubic foot = 0.037 cubic yards
Example: A patio that is 12 feet long, 10 feet wide, and 4 inches deep. First, convert 4 inches to feet: 4 / 12 = 0.33 feet. Then multiply: 12 x 10 x 0.33 = 39.6 cubic feet. Divide by 27: 39.6 / 27 = 1.47 cubic yards. With the 10% waste factor (covered below), you would order about 1.6 cubic yards.
Bags vs. Ready-Mix: Which Should You Use?
The size of your project determines whether you buy bagged concrete from a hardware store or order a ready-mix delivery truck. The crossover point is roughly 1 cubic yard. Below that, bags are practical. Above that, a truck saves time, labor, and often money.
Bags Per Cubic Yard
40-lb bags: ~90 bags per cubic yard (each yields ~0.011 cu yd)
60-lb bags: ~60 bags per cubic yard (each yields ~0.017 cu yd)
80-lb bags: ~45 bags per cubic yard (each yields ~0.022 cu yd)
Bagged concrete works well for post holes, small footings, patching, and projects under half a cubic yard. You control the pace, mix only what you need, and do not need to coordinate a delivery window. The downside: mixing 45 or more 80-pound bags by hand is exhausting, slow, and produces less consistent results than a truck.
Ready-mix delivery is the way to go for slabs, driveways, large patios, and any pour over 1 cubic yard. The concrete arrives pre-mixed to exact specifications, and a full truck can deliver 8-10 cubic yards. Most suppliers have a minimum order (typically 1 cubic yard) and may charge a short-load fee if you order less than the truck's capacity. Call ahead to confirm pricing and minimums in your area.
Concrete Amounts for Common Projects
These estimates assume standard dimensions and a 4-inch depth unless otherwise noted. Use them as quick reference points, then adjust for your specific measurements.
Project Estimates (Before Waste Factor)
10 x 10 patio slab (4 in): 1.23 cubic yards (~56 bags of 80 lb)
12 x 20 garage slab (6 in): 4.44 cubic yards (order ready-mix)
3 x 30 sidewalk (4 in): 1.11 cubic yards (~50 bags of 80 lb)
4 x 50 driveway section (6 in): 3.70 cubic yards (order ready-mix)
Single post hole (10 in diameter, 42 in deep): 0.08 cubic yards (~4 bags of 80 lb)
Continuous footing (12 in wide, 8 in deep, 40 ft long): 0.99 cubic yards (~45 bags of 80 lb)
8 x 10 shed pad (4 in): 0.99 cubic yards (~45 bags of 80 lb)
For circular shapes like post holes and tube footings, the formula changes slightly. You calculate the area of a circle (pi times radius squared) and then multiply by depth. A 10-inch diameter post hole that is 42 inches deep: radius = 5 inches = 0.417 feet, area = 3.14159 x 0.417 x 0.417 = 0.545 sq ft, depth = 42 inches = 3.5 feet, volume = 0.545 x 3.5 = 1.91 cubic feet = 0.07 cubic yards. For a fence with 20 post holes, that adds up to about 1.4 cubic yards total.
The 10% Waste Factor
Always order more concrete than your exact calculation calls for. Concrete sticks to the inside of mixers and wheelbarrows, forms are never perfectly level, ground settles unevenly, and small spills are inevitable. The industry standard is to add 10% extra to your calculated volume.
Applying the Waste Factor
Calculated volume: 2.5 cubic yards
Waste factor (10%): 2.5 x 0.10 = 0.25 cubic yards
Total to order: 2.5 + 0.25 = 2.75 cubic yards
In some cases you should increase the waste factor beyond 10%. If the ground is uneven or has not been compacted, the concrete fills low spots you did not account for. If your forms are not perfectly straight, extra concrete fills the gaps. For rough terrain or complex shapes, consider a 15% waste factor to be safe. Running short mid-pour is far worse than having a small amount left over.
Tips for Ordering Concrete
7 Rules for Getting Your Concrete Order Right
1. Measure twice, calculate once. Go back and re-measure your forms after you build them. Dimensions change during construction, and a half-inch difference across a 20-foot slab adds up fast.
2. Always add 10% for waste. This is non-negotiable. Experienced contractors order 10% extra on every job because running short during a pour can ruin the entire project.
3. Round up, not down. If the math says 2.3 cubic yards, order 2.5. Ready-mix trucks can deliver in quarter-yard increments. For bags, round up to the nearest full bag.
4. Know your PSI rating. Standard residential concrete is 3,000-4,000 PSI. Driveways and garage floors should be at least 4,000 PSI. Post holes and non-structural pours can use 2,500 PSI. Specify this when ordering ready-mix.
5. Schedule the pour for mild weather. Concrete cures best between 50-80 degrees Fahrenheit. Extreme heat causes it to set too fast; freezing temperatures weaken the final product. Plan your project around the forecast.
6. Have enough help on pour day. For ready-mix deliveries, you typically have 60-90 minutes to place, spread, and finish the concrete before it becomes unworkable. A 3-yard pour is a two to three person job at minimum.
7. Compact the base first. Pour concrete over compacted gravel, not loose dirt. A 4-inch layer of compacted gravel provides drainage and prevents the slab from cracking as the soil shifts. Skipping this step is the most common cause of cracked slabs.
Planning a project that also involves gravel or crushed stone for your base layer? The gravel calculator estimates how much base material you need before the concrete goes on top.
Common Concrete Mistakes to Avoid
Forgetting to convert inches to feet. This is the number one math error in concrete calculations. A 4-inch slab is 0.33 feet, not 4 feet. If you multiply by 4 instead of 0.33, your volume calculation will be 12 times too high and you will order a wildly incorrect amount.
Not accounting for the base layer. If you excavate 8 inches deep but plan for 4 inches of gravel base and 4 inches of concrete, your concrete depth is 4 inches, not 8. Subtract the gravel layer from your excavation depth to get your true concrete thickness.
Mixing too many bags at once. When working with bagged concrete, mix only as much as you can place in 15-20 minutes. Concrete that sits in the wheelbarrow too long begins to set and becomes difficult to work. It is better to mix in small batches and maintain a steady pace.
Skipping reinforcement on slabs. Any concrete slab over 4 feet in any direction should include wire mesh or rebar for reinforcement. Without it, the slab will eventually crack as the ground shifts. Reinforcement does not prevent all cracking, but it holds the slab together and prevents cracks from widening.
Ignoring the curing process. Concrete reaches about 70% of its final strength in 7 days and full strength in 28 days. Keep it moist for the first 3-5 days by spraying with water or covering with plastic sheeting. Walking on a slab after 24 hours is fine, but do not drive on a driveway or load heavy equipment onto a slab for at least 7 days.
Calculate Your Concrete Now
Concrete Calculator
Enter your project dimensions and get an instant estimate of cubic yards, number of bags, and ready-mix quantity with waste built in.
More Useful Tools
Gravel Calculator
Estimate gravel, crushed stone, or sand for base layers and landscaping.
Calculate →Square Footage Calculator
Quick area calculations for any shape, room, or outdoor space.
Calculate →Flooring Calculator
Calculate materials for hardwood, tile, laminate, or concrete floors.
Calculate →The Bottom Line
Calculating concrete comes down to multiplying length by width by depth in feet, dividing by 27 to get cubic yards, and adding 10% for waste. Whether you are pouring a small patio with bags from the hardware store or scheduling a ready-mix truck for a full driveway, the math is the same. Measure your forms carefully, convert inches to feet before multiplying, round up to the nearest quarter yard, and always have extra on hand. A few minutes of math or a quick run through the concrete calculator saves you from the worst-case scenario: running short halfway through a pour with concrete already setting in your forms.