How to measure a slab
4 measurements, and two of them are where the money goes wrong. Take these, then run them through the calculator.
The 4 measurements
Width and length are the inside faces of the form, because that is where the concrete stops. The diagonals are not used in the arithmetic — they are the check that the first two are telling the truth.
-
Width, in feet
Inside face of the form to inside face, across the short dimension. Not the outside of the lumber, and not the string line you set the forms to. Decimals are fine: 20 ft 6 in is 20.5.
-
Length, in feet
The same, along the long dimension. Measure it at both ends. If the two readings differ by more than an inch the form is not parallel, and the pour will take the larger of the two.
-
Thickness, in inches, from the top of the gravel
Compact the base first, then measure from the top of the gravel up to the finished surface you have marked on the form. Take it in at least four places, including the middle. This is the measurement that decides everything: on 400 ft², one inch is 1.23 cubic yards.
-
Both diagonals, as a check
Corner to opposite corner, both ways. Equal diagonals mean a square form and a slab that is genuinely width times length. Unequal ones mean you are pouring a parallelogram, the area is smaller than the arithmetic says, and the form wants racking before the truck arrives.
Where the tape starts
Yellow line, top of the compacted gravel: that is zero. Dashed line, the dirt: that is not. Every thickness on this site is measured on the yellow line.
The two errors that cost money
1. Measuring thickness from the ground
The gravel base is not concrete. Measure down from the finished height to the dirt and you have counted four inches of crushed stone as four inches of mix. It is the most common error on this page's subject, and it always goes the same direction: you over-order and pay for concrete that was never going anywhere.
One inch too deep on 20 × 20 ft = 1.23 yd³ = about $240
2. Measuring thickness in one place
A base that dips in the middle makes the slab thicker than nominal over most of its area, and the one reading you took at the edge does not know that. This error goes the dangerous direction: you under-order, the truck leaves half a yard short, and you finish with a cold joint across the middle of the slab or pay a second delivery for a wheelbarrow of concrete.
Half an inch of average dip on 20 × 20 ft = 0.62 yd³ short = about $120
Worked through, once
- Measured
- 20 × 20 ft and 4 inches, diagonals within a quarter of an inch
- Volume
- 20 × 20 × (4 ÷ 12) = 133.3 ft³ = 4.94 yd³
- Order
- 5.43 yd³ with the 10% allowance — 1 truck load, a full one being typically about 10 yd³
- Or in bags
- 223 × 80 lb, or 297 × 60 lb, both rounded up
- The call
- Past 60 bags, so ready-mix — see the blade on the calculator
Before the truck is booked
Questions
Where exactly do I measure thickness from?
From the top of the compacted gravel base to the finished surface of the slab. The gravel is not concrete, and the mistake is expensive: one inch of it counted as concrete on a 20 × 20 ft slab is 1.23 cubic yards. Compact the base, set a stake to the finished height, and measure down from that.
How much extra should I order?
10 percent, and where it appears depends on what you are buying. Ready-mix carries it inside the ordered figure, because a truck will not come back for a quarter yard. Bag counts do not: the headline count is the volume, and the plus figure sits beside it, because bags are bought one at a time and an unopened one goes back. Either way the allowance covers spillage, a base that is not perfectly flat and the thickness you gain at the edges of the form.
Bags or ready-mix — where is the line?
Around 60 bags of 80 lb, which is 36 cubic feet or 1.33 cubic yards. Under that you can mix it in a day with a rented mixer and one helper, and you pay only for what you open. Over it you are buying several pallets, handling two tons or more by hand, and racing the set on the first batch you placed. Some plants charge a short-load fee below about 3 yards, so ask when you book.
How many 80 lb bags are in a cubic yard?
45, and you buy 45. An 80 lb bag of mix yields about 0.60 cubic feet once water is in it, and a cubic yard is 27 cubic feet. A 60 lb bag yields about 0.45 cubic feet, so 60 of those make a yard. Every bag count on this site is rounded up, because a store will not sell you nine tenths of a bag.
What is a mixer-truck load?
A full truckload is typically about 10 cubic yards, and that is the figure this site plans with. Ask when you book: drums vary, and a plant will tell you what it is sending. The load count here is worked from the ordered volume — the one that already includes the 10% allowance — because that is what actually has to fit on the truck.
Bag yields: QUIKRETE Concrete Mix data sheet No. 1101. Price basis: Concrete Financial Insights weighted average selling price for 2025, escalated with U.S. Bureau of Labor Statistics PPI series PCU327320327320 (2026-07). Prices vary by region — enter your own quote above.
Updated 2026-08-25