How Much Concrete For A 20x20 Slab 4 Inches Thick

Before you order a truck or stack bags in the driveway, you need a realistic quantity for how much concrete for a 20x20 slab 4 inches thick. Too little concrete means a cold joint halfway through the pour, a second trip, and a weak seam; too much means bags or a short-load fee you did not need. This guide walks through a 20 ft by 20 ft slab, 4 in thick with the volume formula, the typical bag yields from our material data, and a 10% waste allowance, and it ends with a range you can actually order against. Everything here is an estimate — measure twice, check the bag label, and buy a little extra for the parts of the job you cannot see until the forms come off.

The calculator on this page asks for the same inputs the worked example uses: the dimensions of a 20 ft by 20 ft slab, 4 in thick, the depth, and the bag size. It returns a range, because the honest answer is a range — the low end assumes a flat base and no spillage, and the high end assumes the usual 10% waste. Plan your order on the high end. A spare bag costs little; a short pour costs a second delivery, a cold joint, and a visible repair.

How the calculation works

The calculator needs the shape of the pour, the depth, and the bag size you plan to buy, and each one changes the answer. For a rectangular pour the volume is length times width times depth, all in feet, divided by 27 to get cubic yards — one cubic yard is 27 cubic feet (0.76 cubic metres). Depth is where most mistakes happen: 4 in (10 cm) is 0.333 ft, 6 in (15 cm) is 0.5 ft, so always convert inches to feet before multiplying. A slab that is 10 ft by 10 ft at 4 in thick is 100 times 0.333, or 33.3 cubic feet, which is about 1.2 cubic yards before waste. The same slab at 6 in thick is half as much again, about 1.9 cubic yards, which is why the thickness in the keyword matters as much as the footprint.

Once the volume is known, it is converted to bags. Our material data gives typical yields of about 45 80 lb (36.3 kg) bags per cubic yard and about 60 60 lb (27.2 kg) bags per cubic yard, because an 80 lb bag yields about 0.6 cubic feet and a 60 lb bag about 0.45 cubic feet — those are typical values, and the bag label on the exact product you buy governs. On top of that, add the typical waste allowance of about 10% for spillage, an uneven subgrade, and the concrete that stays in the mixer and the wheelbarrow. The worked example below shows the low end with no waste and the high end with the full 10% allowance, so you can see the range instead of a single number that pretends the pour will be perfect.

A 20 by 20 slab at 4 in is a large pour — 400 square feet, about 5 cubic yards — and at this size bagged mix stops being practical for most people. The volume is 400 square feet times a third of a foot, which is about 133 cubic feet. A ready-mix truck is the usual route, and the pour needs enough help to screed, float and finish before the concrete starts to set. If the slab has a thickened edge or a footing around the perimeter, that edge is extra volume on top of the slab itself.

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

Here is the full arithmetic for a 20 ft by 20 ft slab, 4 in thick.

Step 1 — convert to feet. Length 20 ft. Width 20 ft. Depth 4 in ÷ 12 = 0.333 ft.

Step 2 — volume in cubic feet. 20 × 20 × 0.333 = 133.3 cubic feet.

Step 3 — convert to cubic yards. 133.3 ÷ 27 = 4.94 cubic yards, or about 4.9 cubic yards.

Step 4 — bags, low end. At the typical yield of about 45 80 lb bags per cubic yard: 4.94 × 45 = 222.2 bags, rounded up to 223 bags. With 60 lb bags: 4.94 × 60 = 296.3, rounded up to 297 bags.

Step 5 — bags, high end with the 10% waste allowance. 4.94 × 1.10 = 5.43 cubic yards, or 245 80 lb bags (326 60 lb bags). Order 223–245 80 lb bags and keep one or two spare; a slab that runs short at the far corner leaves a patch that will always show.

Cost guidance

Budgeting for how much concrete for a 20x20 slab 4 inches thick means pricing the range, not a single number. Take the worked-example quantity (4.9–5.4 cubic yards, or about 223–245 80 lb bags) and price it twice: once at the low end of the range and once at the high end, using the current shelf price per bag at the store where you will actually buy, or the per-cubic-yard ready-mix quote from a local supplier. The difference between the two totals is your contingency, and it is usually smaller than the cost of running short mid-pour. Two cost traps catch first-timers: ordering the low end and then paying a second delivery charge for the few bags that finish the job, and forgetting the extras — formwork, stakes, rebar or mesh, a float, and curing compound or plastic sheeting. Compare the total delivered cost, not the per-bag price, and keep one or two unopened bags for patching; an opened bag that gets damp is wasted money.

If the job grows while you measure — a footing that turns out longer than the plan, a sidewalk that needs an extra section — redo the volume from the new dimensions rather than guessing a percentage on top of the old total. Concrete quantities scale with volume, not with length alone, so a small change in depth changes the total more than most people expect. And if you are comparing bagged mix with ready-mix, remember the comparison only works when both quotes cover the same volume including waste, the same delivery distance, and the same extras; a cheap per-bag price with an expensive delivery is not a cheap job.

Step by step: measure, calculate, order

1. Measure twice. Measure the slab length and width in feet, and the depth in inches at six or more places across the base. Measure each run separately — a footing that turns a corner is two runs, not one — and write every dimension down in feet and inches before you convert anything.

2. Convert and calculate. Convert inches to feet (divide by 12), then apply the volume formula for your shape: length times width times depth divided by 27 for a rectangle, or pi times radius squared times depth for a round tube or post hole. Use the calculator on this page to check your arithmetic, and run it twice with the low and high depth if your excavation is uneven.

3. Add waste and round up. Add the typical 10% waste allowance for spillage and an uneven subgrade, then round up to whole bags or to the nearest quarter cubic yard for a truck. A bag cannot be bought in fractions, and a pour that stops short is far more expensive than one spare bag.

4. Order and check the delivery. Order the high end of the bag range below, all the same mix, all the same product and mix, and check the bag labels against the yields the calculator assumed. If you are ordering ready-mix, confirm the slump, the mix strength and the delivery window, and make sure the truck can reach the pour.

5. Before you pour. Set and brace the forms, check the depth with a straightedge in several places, wet the subgrade lightly so it does not steal water from the mix, and have your screed, float and curing cover within reach. Concrete waits for nobody once it is mixed.

Common mistakes to avoid

Three mistakes ruin more small concrete jobs than anything else. First, using the nominal depth instead of the real one — a slab formed with 2x4s is often poured a little deeper than 4 in once the base settles, so measure after the base is compacted. Second, mixing bag counts from two different bag sizes: 80 lb and 60 lb bags yield different volumes, so pick one size and do all the arithmetic in that size. Third, pouring without a plan for finishing and curing — concrete that dries too fast cracks, and concrete that is not screeded level cannot be fixed later. Have the tools, the help and the curing cover ready before the first bag is mixed, and never add extra water to make a stiff mix easier to place; it weakens the finished pour.

Frequently asked questions

How much concrete do I need for a 20x20 slab 4 inches thick?

Multiply length times width times depth and divide by 27. A 20 ft by 20 ft slab at 4 in thick is 133.3 cubic feet, or about 4.9 cubic yards before waste — 4.9–5.4 cubic yards with the typical 10% allowance, or 223–245 80 lb bags if you were to mix it by hand.

Should I use bags or ready-mix for a 20x20 slab?

Ready-mix, in almost every case. Bagged mix would mean about 223–245 80 lb bags to mix and place before the first part starts to set, which is not realistic for one person and barely realistic for a crew. A ready-mix truck delivers about 4.9 cubic yards in one load, and the pour is a single continuous placement with no cold joints.

How many control joints does a 20x20 slab need?

Joints every 8–10 ft at most, which divides a 20 by 20 slab into roughly nine sections. The joints are cut or tooled into the finished surface and do not change the volume estimate. A slab this size without joints will crack, and the cracks will run where they want to rather than where you planned them.

How much does a 20x20 slab weigh?

Concrete weighs roughly 140–150 lb per cubic foot, so a 20 by 20 slab at 4 in thick — about 133 cubic feet — weighs roughly 18,600–20,000 lb, or about 9–10 tons. That is why the base matters so much: the ground underneath has to carry that weight without settling, and a base that settles takes the slab down with it.

Can I pour a 20x20 slab in sections?

Yes, and many large slabs are poured that way, with a joint between sections. Work out the volume per section and order for the sections you plan to pour that day, so each section is a single continuous pour with no cold joint inside it. Plan the joints to fall at regular intervals so the finished slab looks deliberate.

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.