How Much Concrete Do I Need? Calculator Guide

Calculate how much concrete you need by multiplying length (feet) x width (feet) x thickness (inches / 12), then dividing by 27 to convert cubic feet to cubic yards. A 10×10 slab at 4 inches thick needs 1.23 cubic yards. A 20×20 slab at 4 inches needs 4.94 cubic yards. A 40×60 slab at 4 inches needs 29.63 cubic yards. Always order 5% to 10% more than your calculated amount to account for waste, uneven sub-grade, and slightly thicker areas. This guide walks through the concrete calculation formula, provides quick-reference tables for common sizes, and explains how to order correctly.

The Concrete Volume Calculation Formula

The formula for calculating concrete volume is straightforward. Multiply the slab length in feet times the width in feet times the thickness in feet (convert inches to feet by dividing by 12). This gives you cubic feet. Divide cubic feet by 27 to convert to cubic yards, which is how ready-mix concrete is sold and delivered.

Formula: (Length x Width x Thickness in feet) / 27 = Cubic Yards

Example: A 20×20 slab at 4 inches thick. Length = 20 feet. Width = 20 feet. Thickness = 4 inches / 12 = 0.333 feet. Volume = 20 x 20 x 0.333 = 133.2 cubic feet. Cubic yards = 133.2 / 27 = 4.93 cubic yards. Order 5.5 cubic yards (rounding up with 10% extra).

For irregular shapes, break the area into rectangles, calculate each section separately, and add the volumes together. For circular slabs (round patios, fire pit pads), use: pi x radius squared x thickness in feet / 27. A 12-foot diameter circle at 4 inches thick: 3.14 x 6 x 6 x 0.333 / 27 = 1.40 cubic yards.

Quick Reference: Concrete Needed by Common Slab Sizes

At 4 inches thick: 10×10 = 1.23 yards. 10×12 = 1.48 yards. 12×12 = 1.78 yards. 12×16 = 2.37 yards. 16×16 = 3.16 yards. 16×20 = 3.95 yards. 20×20 = 4.94 yards. 20×24 = 5.93 yards. 20×30 = 7.41 yards. 24×24 = 7.11 yards. 24×30 = 8.89 yards. 30×30 = 11.11 yards. 30×40 = 14.81 yards. 40×40 = 19.75 yards. 40×60 = 29.63 yards.

At 5 inches thick: multiply the 4-inch values by 1.25. At 6 inches thick: multiply the 4-inch values by 1.50. A 20×20 slab at 6 inches needs 7.41 cubic yards (4.94 x 1.5).

These calculations assume a flat slab with uniform thickness. Slabs with thickened edges (monolithic foundations) need additional concrete for the perimeter footers. Add the footer volume separately: footer width (feet) x footer depth (feet) x perimeter length (feet) / 27.

Coverage Rates: One Cubic Yard of Concrete

One cubic yard of concrete covers different square footage depending on slab thickness. At 4 inches thick, one yard covers 81 square feet. At 5 inches thick, one yard covers 64.8 square feet. At 6 inches thick, one yard covers 54 square feet. At 8 inches thick, one yard covers 40.5 square feet.

These coverage rates let you quickly estimate how much concrete you need without doing the full calculation. A 300 square foot patio at 4 inches thick needs approximately 300 / 81 = 3.7 cubic yards. Add 10% and order 4.1 yards, which you would round to 4.5 yards from the batch plant.

Coverage rates also help you estimate cost quickly. At $140 per cubic yard, a 4-inch slab costs approximately $1.73 per square foot in material alone ($140 / 81 square feet). At 6 inches thick, the material cost per square foot rises to approximately $2.59 ($140 / 54 square feet).

How to Calculate Concrete for Special Shapes

For sidewalks and walkways, calculate as a long, narrow rectangle. A 4-foot wide, 50-foot long sidewalk at 4 inches thick: 4 x 50 x 0.333 / 27 = 2.47 cubic yards. Order 2.75 yards.

For steps, calculate each step as a separate rectangular volume. A step that is 36 inches wide, 12 inches deep, and 7 inches tall: 3 x 1 x 0.583 / 27 = 0.065 cubic yards per step. A set of 5 steps needs 0.325 cubic yards. Add the landing pad volume separately.

For footers and foundations, calculate the perimeter volume. A 30×40 building with 16-inch wide, 12-inch deep footers: perimeter = 140 linear feet. Volume = 140 x 1.33 (16 inches) x 1.0 (12 inches) / 27 = 6.9 cubic yards for footers alone. Add the slab volume for the total pour.

For concrete columns and piers, calculate as cylinders. Volume = pi x radius squared x height / 27. A 12-inch diameter, 36-inch deep pier: 3.14 x 0.5 x 0.5 x 3 / 27 = 0.087 cubic yards per pier. Twenty piers need 1.74 cubic yards.

Why You Should Order Extra Concrete

Always order 5% to 10% more concrete than your calculated amount. The sub-grade is never perfectly flat, forms may bow slightly, and the slab thickness will vary across the pour. These variations mean you use more concrete than the theoretical calculation predicts.

Running short during a pour is a serious problem. Once you start pouring, the concrete already placed is setting. If you need an additional half yard and the truck has to make a second trip, the new concrete may not bond properly with the partially set first pour, creating a cold joint.

The cost of ordering extra concrete is small compared to the risk of running short. An extra half yard at $140 is $70 in material. A cold joint repair or a second mobilization fee from the batch plant costs $200 to $500. Over-ordering by a small amount is always the right financial decision.

Most batch plants will not charge you for concrete returned in the truck (they recycle it), and some will adjust the final invoice to reflect only what was poured. Ask your supplier about their return policy when placing the order.

How to Order Ready-Mix Concrete

Call the batch plant at least 2 to 3 days before your pour date. Provide the following: total cubic yards needed, concrete mix strength (3,000 PSI for residential, 4,000 PSI for heavy-duty), any special requirements (air entrainment, fiber reinforcement, color additives), delivery address, and desired delivery time.

A standard mixer truck holds 8 to 10 cubic yards. If you need less than 3 to 5 yards (varies by supplier), expect a short-load fee of $50 to $100. If you need more than 10 yards, the batch plant will schedule multiple trucks to arrive in sequence.

Schedule delivery for early morning on the pour day. This gives you the maximum working time before afternoon heat accelerates the setting process. Have your forms, reinforcement, and crew ready before the truck arrives. The truck driver will start a timer when concrete begins flowing, and most plants allow 5 to 7 minutes per yard for unloading before charging overtime ($2 to $5 per minute).

For bagged concrete (small DIY projects), one 80-pound bag yields approximately 0.6 cubic feet, which means you need 45 bags per cubic yard. A 4×4 slab at 4 inches thick (5.33 cubic feet) needs 9 bags of 80-pound mix. At $5 to $6 per bag, that is $45 to $54 in material.

Concrete Calculator vs Manual Calculation

Online concrete calculators and mobile apps do the same math described in this guide. They accept your slab dimensions and thickness, then output the cubic yardage needed. These tools are convenient for quick estimates and eliminate arithmetic errors.

Manual calculation using the formula (L x W x T in feet / 27) gives you the same result and helps you understand the relationship between slab dimensions and concrete volume. This understanding is valuable when comparing quotes from contractors and verifying that the quoted yardage matches your project dimensions.

Whichever method you use, the critical step is adding the 5% to 10% overage before placing your order. No calculator accounts for sub-grade variation, form flex, and real-world pour conditions. Build the overage into your final order quantity.

Frequently Asked Questions

How much concrete do I need for a 10×10 slab?

A 10×10 concrete slab at 4 inches thick requires 1.23 cubic yards of concrete. Order 1.5 cubic yards to account for waste and sub-grade variation. At a ready-mix price of $140 per cubic yard, the concrete material costs approximately $210. For DIY with bagged concrete, you need approximately 56 bags of 80-pound mix at $5 to $6 each ($280 to $336).

How much concrete do I need for a 20×20 slab?

A 20×20 concrete slab at 4 inches thick requires 4.94 cubic yards. Order 5.5 cubic yards with 10% overage. At 5 inches thick, you need 6.17 cubic yards (order 7 yards). At 6 inches thick, you need 7.41 cubic yards (order 8 yards). Ready-mix concrete at $140 per yard puts material cost for a 4-inch thick 20×20 slab at approximately $770.

How do you calculate cubic yards of concrete?

Multiply length (feet) x width (feet) x thickness (convert inches to feet by dividing by 12). This gives you cubic feet. Divide cubic feet by 27 to get cubic yards. Example: 12 feet x 16 feet x (4 inches / 12) = 64 cubic feet / 27 = 2.37 cubic yards. Add 5% to 10% extra and round up for your order quantity.

How many bags of concrete do I need?

One 80-pound bag of concrete yields approximately 0.6 cubic feet. Divide your total cubic feet by 0.6 to get the number of bags needed. A 4×4 slab at 4 inches thick (5.33 cubic feet) needs 9 bags. A 10×10 slab at 4 inches thick (33.3 cubic feet) needs 56 bags. For projects requiring more than 30 bags, ready-mix delivery is more cost-effective and produces better results.

What does a yard of concrete cost?

A cubic yard of standard 3,000 PSI ready-mix concrete costs $120 to $165 in 2026, with the national average at approximately $140 per yard. Delivery within 10 to 20 miles is usually included. Short-load fees of $50 to $100 apply for orders under 3 to 5 yards. Saturday delivery adds $100 to $200 in surcharges.

How much extra concrete should I order?

Order 5% to 10% more concrete than your calculated volume. For a calculated need of 5 cubic yards, order 5.5 yards. The extra material accounts for uneven sub-grade, form flex, and slab thickness variations across the pour. Running short during a pour creates cold joints and requires expensive second trips. The cost of extra concrete ($70 to $140 per extra yard) is far less than the cost of a short pour.

Can I use a concrete calculator app?

Concrete calculator apps and online tools accurately compute cubic yardage from your slab dimensions. They are convenient for quick estimates. Always verify the app's output matches the manual formula (L x W x thickness in feet / 27) and add 5% to 10% overage before ordering. No calculator accounts for real-world variations in sub-grade and form flex.