★ CONSTRUCTION · CONCRETE · CALCULATORS
A 20 ft × 12 ft driveway, 4 inches thick, needs 2.96 cubic yards of concrete: 178 60-lb bags or 134 80-lb bags, or $444.44 of ready-mix against $603.00 of bags — ready-mix is cheaper by $158.56. Enter your size and prices below, and the calculator works out the yards, the bags with waste, and which way to buy it costs less.
20 ft × 12 ft × 4 in is 80.00 cubic feet, which is 2.96 cubic yards. That is 178 60-lb bags or 134 80-lb bags — 196 or 147 with 10% waste — and ready-mix at $150.00 a yard costs $444.44 against $603.00 for bags.
A driveway is a slab, and a slab is length times width times thickness. The only trick is the units: thickness is in inches, so it is divided by 12 to get feet, and the volume in cubic feet is divided by 27 to get cubic yards, because a cubic yard is 3 ft × 3 ft × 3 ft. Bags are the volume divided by the yield of one bag — 0.45 cubic feet for a 60-lb bag, 0.60 for an 80-lb bag — rounded up, because you cannot buy part of a bag. Ready-mix is the cubic yards times the price per yard. At the defaults the two ways to buy it are $158.56 apart, and the ready-mix truck wins.
Setup: driveway 20 ft × 12 ft; thickness 4 in; 60-lb bag $4.50; 80-lb bag $4.50; ready-mix $150.00 a cubic yard; waste 10%.
Line 1 — volume in cubic feet. 20 × 12 × 4 / 12 = 80.00 ft³.
Line 2 — cubic yards. 80.00 / 27 = 2.96 yd³.
Line 3 — 60-lb bags. 80.00 / 0.45 = 177.78 → 178 bags.
Line 4 — 80-lb bags. 80.00 / 0.60 = 133.33 → 134 bags.
Line 5 — waste. 177.78 × 1.1 = 195.56 → 196 60-lb bags; 133.33 × 1.1 = 146.67 → 147 80-lb bags.
Line 6 — ready-mix cost. 2.96 × $150.00 = $444.44.
Line 7 — bag cost. 134 × $4.50 = $603.00 for 80-lb bags; 178 × $4.50 = $801.00 for 60-lb bags.
Line 8 — the verdict. $603.00 − $444.44 = $158.56 for ready-mix. With waste the bags cost $661.50, so the gap grows to $217.06.
Small 12 ft × 10 ft, 4 in: 40.00 ft³, which is 1.48 yd³. That is 89 60-lb bags or 67 80-lb bags — 98 or 74 with 10% waste, which cost $441.00 or $333.00. Ready-mix is $222.22 against $301.50 of bags, so ready-mix still wins, by $79.28. But most suppliers charge a short-load fee under about 3 yards, which can erase that saving — ask before ordering.
Big 24 ft × 20 ft, 6 in: 240.00 ft³, which is 8.89 yd³. That is 534 60-lb bags or 400 80-lb bags — 587 or 441 with 10% waste, which cost $2,641.50 or $1,984.50. Ready-mix is $1,333.33 against $1,800.00 of bags, so ready-mix wins by $466.67. At this size bags are not a serious option: 400 bags is a full day of mixing by hand.
Thicker, 20 ft × 12 ft at 5 in: 100.00 ft³, which is 3.70 yd³. That is 223 60-lb bags or 167 80-lb bags. Ready-mix is $555.56 against $751.50 of bags. One extra inch costs $111.12 of ready-mix.
Thicker, 20 ft × 12 ft at 6 in: 120.00 ft³, which is 4.44 yd³. That is 267 60-lb bags or 200 80-lb bags. Ready-mix is $666.67 against $900.00 of bags. Two extra inches over the 4-inch base cost $222.23 of ready-mix.
Metric, 6 m × 3.5 m × 0.1 m: 2.10 m³, which is 74.16 ft³ and 2.75 yd³, or 124 80-lb bags. A cubic metre is 35.3147 cubic feet, so the conversion is 2.10 × 35.3147 = 74.16.
Reverse — what 50 bags pour: 50 80-lb bags is 30.00 ft³, and a 12-ft-wide driveway 4 inches thick is 4.00 ft³ per foot of length, so 30.00 / 4.00 = 7.50 ft. Fifty bags pour seven and a half feet of driveway — enough for a patch, not a driveway.
4 inches is the standard for cars. A 4-inch slab on a compacted gravel base carries passenger cars and light trucks without cracking, and it is what the defaults on this page assume. 5 to 6 inches is for heavy loads: delivery trucks, RVs, boats on trailers, and anything that parks a heavy axle on the same spot every day. The extra concrete is cheap insurance — going from 4 to 6 inches on a 20 ft × 12 ft driveway adds $222.23 of ready-mix and roughly doubles the slab’s load capacity. Whatever the thickness, put steel in it: welded wire mesh or rebar keeps cracks tight and the slab in one piece, and a 4,000 PSI mix is the usual pick for driveways in freeze-thaw climates, because it resists the scaling that salt and ice cause. None of that changes the volume on this page — the steel and the mix strength are separate line items — but order the concrete for the thickness the loads need, not the thickness the budget wants.
The defaults are a 20 ft × 12 ft driveway, 4 inches thick, with 60-lb and 80-lb bags at $4.50 each, ready-mix at $150.00 a cubic yard, and 10% waste. The bag counts are the exact volume divided by the nominal yield, rounded up: a 60-lb bag is taken as 0.45 cubic feet and an 80-lb bag as 0.60 cubic feet. The ready-mix cost is the unrounded cubic yards times the price per yard.
Rounding convention, stated plainly: volume is rounded to two decimals — 80.00 ft³, 2.96 yd³ — and bags are whole numbers, always rounded up. The waste count is the exact bag count times 1.1, rounded up: for the big driveway that gives 441 80-lb bags, because (240 / 0.60) × 1.1 is 440.00000000000006 in the calculator’s arithmetic, and the ceiling of that is 441. Money is rounded to cents.
| Output | Formula | What it means |
|---|---|---|
| Volume | 20 × 12 × 4 / 12 | 80.00 ft³ of concrete to pour. |
| Cubic yards | 80.00 / 27 | 2.96 yd³ — how ready-mix is sold. |
| 60-lb bags | 80.00 / 0.45, rounded up | 178 bags if you mix by hand. |
| 80-lb bags | 80.00 / 0.60, rounded up | 134 bags if you mix by hand. |
| 60-lb bags with waste | 177.78 × 1.1, rounded up | 196 bags to order. |
| 80-lb bags with waste | 133.33 × 1.1, rounded up | 147 bags to order. |
| Ready-mix cost | 2.96 × $150.00 | $444.44 delivered by truck. |
| 80-lb bag cost | 134 × $4.50 | $603.00 mixed by hand. |
| Ready-mix saving | $603.00 − $444.44 | $158.56 cheaper by truck. |
The math is exact for the numbers you enter: volume is length times width times thickness over 12; cubic yards is volume over 27; bags are volume over the bag yield, rounded up; waste is the bag count times one plus the waste percent, rounded up; costs are the counts times the prices you enter. But the inputs carry the assumptions. Bag yields are nominal — 0.45 and 0.60 cubic feet — and real bags vary a little with the mix and the moisture; a bag that yields 0.58 instead of 0.60 costs you two extra bags on a 134-bag order. Ready-mix is priced at $150.00 a cubic yard here, and local prices vary widely: some markets are $120.00, some are $200.00, and short-load fees, Saturday delivery, and pump charges are all extra. Order 10% extra, as the defaults do: a slab that runs short mid-pour is a cold joint, and a cold joint is a crack waiting to happen. And the subgrade must be compacted before anything is poured — concrete poured on soft ground settles, cracks, and no calculator can fix that. This page counts concrete only: mesh, rebar, forms, gravel base, finishing, and labour are separate costs.
Bright Side Kit. "Concrete Driveway Calculator: Yards, Bags & Total Cost" https://brightoolbox.com/concrete-driveway-calculator/ Accessed October 24, 2026.2.96 for a 20 ft × 12 ft driveway at 4 inches. The volume is 20 × 12 × 4 / 12 = 80.00 cubic feet, and 80.00 / 27 = 2.96 cubic yards. Order 3 yards: suppliers sell whole yards, and the 10% waste allowance covers the rest.
178 60-lb bags or 134 80-lb bags for the 20 ft × 12 ft, 4-inch driveway — 196 or 147 with 10% waste. A 60-lb bag yields 0.45 cubic feet and an 80-lb bag yields 0.60, so 80.00 cubic feet divided by the yield, rounded up, is the count.
Ready-mix, by $158.56 at the defaults. Ready-mix is $444.44 for 2.96 cubic yards against $603.00 of 80-lb bags. Bags only win on pours too small for a truck — but ask about the short-load fee first, because it can erase the saving on a small order.
4 inches for cars, 5 to 6 inches for heavy trucks and RVs. A 20 ft × 12 ft driveway at 5 inches needs 3.70 cubic yards and $555.56 of ready-mix; at 6 inches it needs 4.44 cubic yards and $666.67. Add mesh or rebar, and use a 4,000 PSI mix in freeze-thaw climates.
10%. The defaults order 196 60-lb bags instead of 178, or 147 80-lb bags instead of 134. A slab that runs short mid-pour gets a cold joint, and a cold joint is a crack waiting to happen — the extra concrete is the cheapest insurance on the job.
Yes — mesh or rebar, in every driveway. Steel does not change the volume on this page, but it keeps cracks tight and the slab in one piece when the ground moves. A 4-inch slab for cars usually takes welded wire mesh; a 5- to 6-inch slab for heavy loads usually takes rebar.
Volume in cubic metres is length × width × thickness, all in metres. A 6 m × 3.5 m slab 0.1 m thick is 2.10 m³, which is 74.16 cubic feet and 2.75 cubic yards, or 124 80-lb bags. A cubic metre is 35.3147 cubic feet.
7.50 ft of 12-ft-wide, 4-inch driveway. Fifty 80-lb bags is 30.00 cubic feet, and a 12-ft-wide slab 4 inches thick uses 4.00 cubic feet per foot of length, so 30.00 / 4.00 = 7.50 ft. Enough for a patch, not a driveway.