Concrete Calculator for a 10x20 Slab

A 10 by 20 ft slab, 4 in (10.2 cm) thick starts with one honest question: how much concrete do you actually need? This page answers it for concrete calculator for a 10x20 slab, using the cubic-yard formula and the typical bag yields in our material data. The result on this page is an estimate, not a guarantee: coverage, pack sizes, yields and prices vary by brand, product line, surface and region, and the label on the product you buy governs. Every figure below is shown as a range for exactly that reason, and the calculator on this page asks for your measurements instead of handing you a single magic number.

Slab sizes are the most common concrete question on this site, and the answer is always the same formula with different dimensions — which is why the worked example below shows every step. The rest of this page explains how the calculation works, shows a full worked example with its own measurements and a range result, gives cost guidance as ranges, and finishes with the steps to measure, order and check your delivery. Nothing here is a single exact number, because a single exact number would be dishonest: subgrades are uneven, forms flex, spillage happens, and a bag that yields one volume on paper can yield a little less once it is mixed and placed.

How the calculation works

The calculator needs three inputs, and each one changes the answer. First, the length and width in feet, measured at the longest and widest points of the form — for a rectangle that is a single multiplication, and for an L-shape or a footing run it is the sum of the separate rectangles. Second, the depth in inches, converted to feet by dividing by 12: a 4 in (10.2 cm) slab is one-third of a foot deep, a 6 in (15.2 cm) slab is half a foot, and an 8 in (20.3 cm) slab is two-thirds. Third, the waste allowance: our material data uses about 10% extra for waste, spillage and uneven subgrade as the typical allowance, because forms are never perfectly true and a little concrete always stays in the mixer, the wheelbarrow or the chute.

For a 10 by 20 ft slab the volume at 4 in thick is 66.7 cu ft, which is 2.5 cubic yards before waste. At 6 in thick the same slab would need about 3.7 cubic yards, so depth matters as much as area — an extra two inches of depth adds half as much concrete again.

The formula is short: cubic yards = length (ft) × width (ft) × depth (ft) ÷ 27, because one cubic yard holds 27 cubic feet (0.76 cu m). Bagged mix follows from the same volume: one cubic yard takes about 45 bags of 80 lb (36.3 kg) mix or about 60 bags of 60 lb (27.2 kg) mix, typical yields from our material data — an 80 lb bag yields about 0.6 cu ft and a 60 lb bag about 0.45 cu ft. Multiply the cubic yards by 45 or by 60, add the 10% allowance, and round up to whole bags. Always round up: a pour cannot wait while someone fetches three more bags, and a short load costs far more than the spare bags would have.

Worked example: a 10 by 20 ft slab, 4 in thick

Here is the full arithmetic for a 10 by 20 ft slab, 4 in thick, with every step shown so you can check it against your own project.

Step 1 — area. 10 ft × 20 ft = 200 sq ft (18.6 sq m).

Step 2 — volume. 200 sq ft × 4 in ÷ 12 = 66.7 cu ft.

Step 3 — cubic yards. 66.7 ÷ 27 = 2.5 cubic yards; with the 10% allowance that is 2.7 cubic yards.

Step 4 — bags. 2.5 × 45 = 112 80 lb bags at the low end; 2.7 × 45 = 123 at the high end. In 60 lb bags that is 149–163 60 lb bags.

Result: about 2.5–2.7 cubic yards, or 112–123 80 lb bags — order the high end, and ready-mix usually beats bags once the job passes about a cubic yard. Buy a few extra bags beyond the high end if the subgrade is uneven or the forms are rough, and keep the spares dry — bagged mix that gets damp in storage is wasted money.

Cost guidance

Costs on this page are ranges, labelled typical retail range — check the current shelf price before budgeting. No single national price is quoted, because ready-mix and bagged concrete prices vary by region, supplier, mix strength and delivery distance, and a price that is right at one plant can be wrong at the next. The way to budget is to multiply, not to guess: take the cubic yards from the worked example, multiply by the shelf or quoted price per cubic yard for the mix you have chosen, then add delivery, any short-load fee for small orders, reinforcement such as rebar or mesh, and formwork and finishing tools if you do not own them.

Two cost traps catch first-time buyers. The first is ordering the low end of the range and then paying for a second small delivery — or a short-load fee — for the concrete that finishes the pour. The second is forgetting the extras: reinforcement, formwork, curing compound and finishing tools together are a meaningful share of a small job's cost, and a slab that needs excavation or a gravel base costs more before a single yard is poured. Compare the total, not the per-yard price, and get the quote against the same dimensions and depth you used for the volume count. Ready-mix usually beats bagged mix on price once the job passes about a cubic yard, while bagged mix wins for small repairs where a truck cannot justify the trip.

Step by step: measure, calculate, order

1. Measure twice. Measure the length and width in feet at the longest and widest points, and the depth in inches with a tape held against the form. Forms are rarely perfectly true — measure the depth in several places and use the deepest reading for the order.

2. Choose the mix and the method. Ready-mix suits jobs over about a cubic yard; bagged mix suits small pours and repairs. Check the bag label or the supplier's ticket for the mix strength and the yield per bag before you commit.

3. Use the calculator on this page. Enter the length, width and depth, and the tool returns a range in cubic yards and in bags. Plan on the high end — a pour cannot wait while someone fetches more mix.

4. Order and check the delivery. Order the high end of the range, confirm the delivery time and the access route for the truck, and have the forms, reinforcement and finishing tools ready before the concrete arrives.

5. Before you pour. Wet the subgrade lightly, check the forms are level and braced, confirm the reinforcement is in place, and make sure you have help, wheelbarrows and finishing tools within reach — concrete waits for no one.

Common mistakes to avoid

  • Ordering the low end of the range and then paying a short-load fee or a second delivery for the concrete that finishes the pour.
  • Measuring the depth once, at the shallowest spot, and under-ordering by a tenth of a yard or more.
  • Forgetting the 10% allowance for waste, spillage and uneven subgrade — the subgrade is never as flat as it looks.
  • Pouring without reinforcement, formwork checks or a curing plan — the volume count is only half the job.

Frequently asked questions

How much concrete do I need for a 10 by 20 ft slab, 4 in thick?

About 2.5–2.7 cubic yards including the 10% waste allowance, which is 112–123 80 lb bags or 149–163 60 lb bags. The range exists because subgrades are uneven and a little concrete always stays in the mixer — order the high end.

How many bags of concrete are in a cubic yard?

About 45 bags of 80 lb mix or about 60 bags of 60 lb mix per cubic yard, typical yields from our material data — an 80 lb bag yields about 0.6 cu ft and a 60 lb bag about 0.45 cu ft. Check the bag label on the exact product you buy, because yields vary by mix.

Should I use ready-mix or bagged concrete?

Ready-mix usually wins once the job passes about a cubic yard, because mixing dozens of bags by hand is slow and the mix can vary from batch to batch. Bagged mix wins for small pours and repairs where a truck cannot justify the trip — count the bags first, then decide.

Why add 10% extra concrete?

The 10% allowance in our material data covers waste, spillage and uneven subgrade. Forms flex, the subgrade is never perfectly flat, and a little concrete stays in the mixer, the wheelbarrow and the chute. A short pour costs far more than the spare bags would have.

What should I check before ordering concrete for concrete calculator for a 10x20 slab?

Check the depth in several places and use the deepest reading, confirm the formwork is level and braced, confirm the reinforcement is in place, and confirm truck access and the delivery time. Have help, wheelbarrows and finishing tools ready before the concrete arrives — it waits for no one.

Related calculators

Results are estimates — verify with the product label. Coverage, pack sizes, yields and prices vary by brand, product line, surface and region; the label on the product you buy governs. Cost figures are typical retail ranges — check the current shelf price before budgeting.