How do I calculate yards of concrete?
Multiply length by width by depth in feet to find cubic feet, divide by 27 for cubic yards, and then add a realistic allowance for site and placing loss.
Step-by-step calculation guide
How to Calculate Concrete
Learn the formulas for yards, bags, slabs, footings, and blocks, with practical examples for planning a project.
How to Calculate Concrete Yards
Concrete sold for a larger pour is usually measured in cubic yards. A cubic yard is a volume, not a surface area: it is a cube 3 feet long, 3 feet wide, and 3 feet high. That cube contains 27 cubic feet. To calculate concrete yards for a rectangular area, first make every dimension compatible. Keep length and width in feet. If thickness is in inches, divide it by 12 to express it in feet. Multiply length × width × thickness in feet to find cubic feet, then divide by 27. Written as one line, the formula is length (ft) × width (ft) × thickness (in) ÷ 12 ÷ 27 = cubic yards.
Consider a patio that measures 18 feet by 12 feet and will be 4 inches thick. Change 4 inches to 0.333 feet. The volume is 18 × 12 × 0.333, or about 72 cubic feet. Divide 72 by 27 to get 2.67 cubic yards. That is the geometric amount. Real forms and ground are rarely perfect, and some material remains in tools or is lost during placing. Adding 10% brings the planning quantity to about 2.94 cubic yards. A supplier may sell in quarter-yard increments and may advise a final order of 3 cubic yards. Do not round down before adding the allowance.
Complex plans become easier when divided into basic shapes. Calculate a patio, walkway, landing, and thickened section separately. Add the raw volumes, then apply one extra allowance to the combined total. If sections have different depths, never average them unless their areas are identical; compute each depth on its own. For a circular area, calculate surface area as π × radius², multiply by depth in feet, and follow the same cubic-foot-to-yard step. Record measurements and intermediate answers so a supplier can help check the work. The free concrete yard calculator handles the units, a second rectangular area, bag counts, and metric output when you want an immediate result.
How to Calculate Concrete Bags
Bag calculations begin with the same project volume, but the final step uses product yield rather than 27 cubic feet per yard. Find the required cubic feet for the shape and add the extra amount appropriate for the work. Then divide adjusted cubic feet by the yield printed on one bag. Always round up to a whole bag. Typical planning yields are about 0.30 cubic feet from a 40 lb bag, 0.45 cubic feet from a 60 lb bag, and 0.60 cubic feet from an 80 lb bag. Product formulas differ, so the label on the material you will buy is the best source for the final count.
Suppose a small pad is 4 feet by 4 feet and 4 inches thick. Its volume is 4 × 4 × 0.333 = about 5.33 cubic feet. Add 10% and the adjusted volume is about 5.87 cubic feet. Divide by 0.60 for 80 lb bags: 9.78, which rounds up to 10 bags. For 60 lb bags, 5.87 ÷ 0.45 = 13.04, so buy 14 bags. For 40 lb bags, 5.87 ÷ 0.30 = 19.56, so buy 20. A smaller bag is easier to lift, but it requires more individual batches. The total dry weight and number of mixing cycles should be part of the plan.
Use bags when the quantity is manageable, access is limited, or a delivered load would trigger an uneconomical minimum charge. Before choosing, consider water access, mixing capacity, working time, weather, and the number of people available. Concrete should be placed continuously enough for the project, so dozens of hand-mixed batches can be difficult for one person. Store bags dry and off the ground because moisture can start hardening before use. Measure water according to the product directions rather than making the mix overly wet for easier placing. An overly wet batch can reduce quality and increase shrinkage. The concrete bag calculator compares 40, 60, and 80 lb counts and lets you enter a bag price for a fast material budget.
How to Calculate Concrete for a Slab
A slab is usually a rectangular volume defined by finished length, finished width, and uniform thickness. Measure the inside dimensions of the forms rather than relying only on a drawing. Multiply length by width to find square feet. Multiply that area by thickness in feet to get cubic feet, then divide by 27 for cubic yards. For thickness entered in inches, the compact formula is length × width × thickness ÷ 324. The number 324 combines 12 inches per foot and 27 cubic feet per cubic yard. This shortcut works only when length and width are feet and thickness is inches.
Thickness makes a large difference. A 20 × 20 foot slab at 4 inches thick has 400 square feet of area. Multiply by 4 ÷ 12 to get about 133.33 cubic feet, or 4.94 cubic yards. The same footprint at 6 inches thick contains 200 cubic feet, or 7.41 cubic yards. Moving from 4 to 6 inches increases volume and material by 50%. The required thickness must come from project needs, support conditions, vehicle or equipment loads, and applicable requirements—not from the desired budget. Thickened edges, beams, steps, and sloped sections add volume and should be measured as separate shapes.
Prepare an order by adding every slab section and then applying an allowance for uneven subgrade, form movement, spillage, and finishing loss. Five to ten percent is a common planning range, but a rough excavation may need more. A carefully formed slab on an even base may need less. Confirm supplier increments and minimums rather than assuming the truck will carry the exact decimal result. Also plan the base, drainage, forms, joints, access, placing crew, finishing tools, and curing before material arrives. Quantity is only one part of a successful slab. Use the concrete slab calculator for live yard and bag results. If the volume is known and you are comparing budget items, the concrete slab cost calculator separates material, labor, and delivery.
How to Calculate Concrete for Footings
Footings can be continuous rectangular strips, square bases, or round piers. Identify the shape before choosing a formula. A continuous footing uses length × width × depth. A square post footing uses side × side × depth. A rectangular base uses length × width × depth. A round hole or tube uses π × radius² × depth. Keep all measurements in feet when multiplying, or calculate in inches and divide cubic inches by 1,728 to get cubic feet. After finding cubic feet for one footing, multiply by the number of identical units and divide the total by 27 for cubic yards.
For example, consider six round footings that are 12 inches in diameter and 36 inches deep. The radius is 6 inches, or 0.5 feet. Each circular area is π × 0.5², about 0.785 square feet. Depth is 3 feet, so each footing contains about 2.356 cubic feet. Six contain about 14.14 cubic feet, which is 0.52 cubic yards. With 10% extra, the plan is about 0.58 cubic yards. At a typical 0.60 cubic feet per 80 lb bag, the adjusted volume would require about 26 bags. Check the chosen product yield and round up.
Excavated holes often flare, crumble, or contain loose material, so their true volume can exceed neat dimensions. A form tube makes the upper shaft predictable, but an enlarged base must be calculated separately. Water in a hole, unsuitable soil, and frost conditions need attention before placing. The dimensions are structural decisions: load, soil bearing, frost depth, wind, and local rules can control them. This guide explains quantity only and cannot choose a safe footing. Once required dimensions are established, the concrete footing calculator switches between round and square shapes, reports per-hole volume, totals multiple holes, and estimates 80 lb bags.
How to Calculate Concrete Blocks
Block walls are counted by face area rather than poured volume. Measure the wall length and finished height in feet, then multiply them to get square feet. Subtract only large openings that meaningfully reduce the number of full and cut units. Next, divide net wall area by the nominal face area of one block. A standard nominal 8 × 8 × 16 inch block, including its mortar module, covers about 0.889 square feet. One square foot therefore needs about 1.125 standard blocks. Round the result up and add extra units for cuts, breakage, corners, and layout changes.
As an example, a 30-foot wall that is 6 feet high has 180 square feet of face area. Divide 180 by 0.889 to get about 202.5, so the base count is 203 blocks. Adding 5% gives 213.15, which rounds to 214 blocks. If the wall contains a 3 × 6.67 foot opening, its area is about 20 square feet, but the final reduction may be less than a simple 22- or 23-block subtraction because cut pieces and the layout around jambs still consume units. Draw courses when openings, corners, bond beams, or special units are important.
Nominal size includes the joint; actual manufactured dimensions are commonly about 3/8 inch smaller. Six-inch, eight-inch, and twelve-inch wall thicknesses can share an 8 × 16 inch face, so their face count may be similar even though weight, purpose, and cost differ. Mortar is estimated separately. A broad planning rule is one 80 lb mortar bag for roughly 30 to 40 standard blocks, but joint depth, block width, waste, and technique change use. Grout inside cells is not mortar and needs its own volume estimate. Structural walls also require project-specific details. Use the concrete block calculator to select standard sizes and receive whole-block and mortar-bag results, then confirm special units with the plans and supplier.
Check the Result Before You Order
Write down the original measurements, formulas, unit changes, waste allowance, and rounded order. Recheck the depth because confusing inches and feet creates one of the largest errors. Make sure every separate shape is included only once. Compare the answer with a rough coverage rule or a second calculation method, and ask the supplier to review the quantity when placing an order. Confirm product yield for bags and clarify which fees apply to a delivery. A calculator can make arithmetic consistent, but it cannot see soft soil, bowed forms, thick edges, inaccessible placement areas, or a last-minute plan change. A short field check before purchase is worth far more than correcting a shortage during the pour.
Keep the final worksheet available on site. If dimensions change after excavation or forms are moved, update the affected shape and calculate the total again instead of adding an unsupported guess. Tell the supplier about access limits, placement method, and timing while confirming quantity. For bag work, count what is physically available and inspect storage condition before starting. A clear written plan helps the crew understand which number is geometric volume, which amount covers extra material, and which rounded amount is actually being purchased.
Frequently Asked Questions
How do you calculate concrete yardage?
For a rectangular pour, use length in feet × width in feet × thickness in inches ÷ 12 ÷ 27. Add separate shapes before applying the final extra allowance.
How many bags of concrete do I need?
Divide required cubic feet by the yield printed on the bag and round up. Typical planning yields are about 0.30 ft³ for 40 lb, 0.45 ft³ for 60 lb, and 0.60 ft³ for 80 lb.
How much does concrete cost per yard?
Local material prices vary with market, strength, load size, and delivery. Ask nearby suppliers for a current per-yard quote and confirm which fees are separate.
How do I calculate concrete for a driveway?
Divide the driveway into rectangles, multiply each length and width by thickness in feet, add their cubic feet, divide by 27, and include suitable extra material.
How do I calculate concrete for footings?
Multiply each footing cross-sectional area by its depth. Use π × radius squared for round holes or length × width for square footings, then total all units.