The formula, without the costume
Volume first, in cubic feet. A slab or a continuous footing is length × width × thickness, with thickness converted from inches by dividing by 12. A round hole or column is the area of a circle times depth: π × (diameter ÷ 2)² × depth, all in feet, times the number of holes. Four holes that are 12 inches across and 24 inches deep are 2π cubic feet, about 6.28.
Steps split in two because the concrete is not the same shape. Porch steps on flat ground are a stack of rectangles. The bottom step is as tall as every riser above it. With n steps the sum is width × tread × rise × n(n+1)/2. Steps cut into a slope, with a sloped soffit, are a wedge: width × (n × rise) × (n × tread) / 2. A 3-foot-wide run of 4 steps at a 7 inch rise and an 11 inch tread is 16.04 cubic feet stacked, and 12.83 cubic feet as a wedge. Ordering the wedge for a porch leaves you short.
Bags are the order volume divided by the published yield, then rounded up. Cubic yards are the same volume divided by 27. One yard is 45 bags of any 80 lb mix that yields 0.60 cubic feet. The unrounded count stays in the detail line so a 55.6 does not get read as 55.
Worked example: a 10 by 10 slab, 4 inches thick
10 × 10 × 4/12 = 33.33 cubic feet, which is 1.23 cubic yards. At QUIKRETE’s 0.60 cubic feet per 80 lb bag, that is 55.56 bags, and you buy 56. The same slab at the 60, 50, and 40 lb yields is 75, 89, and 112 bags, still with no allowance.
Add 10% and the order is 36.67 cubic feet. The 80 lb line becomes 62 bags. That is the pair Sakrete shows on its own calculator: the neat yield, and the yield plus 10%. The form above is loaded with the 10% case. Set the allowance to 0 and the sheet drops back to 56.
Past a couple of yards, bags stop making sense. NRMCA CIP 31 says a truck mixer commonly carries about 8 to 12 cubic yards. Call the volume in, including the allowance, and ask the plant what they will actually batch. This page does not invent a price per yard.
What the defaults assume
The allowance starts at 10% because Sakrete prints that tip on its calculator index, and because 10% is the top of the range in NRMCA CIP 8: “4 to 10 percent over plan dimensions for waste, over-excavation and other contingencies.” A formed slab on a compacted subgrade can live at the low end. A rough post hole should not. QUIKRETE’s own calculator page says the yields do not include uneven ground or waste, which is why a brand calculator can be “right” and still leave you a bag short.
| Bag | QUIKRETE 1101 | Sakrete High-Strength | Sakrete 5000 Plus |
|---|---|---|---|
| 40 lb | 0.30 | 0.30 | — |
| 50 lb | 0.375 | not sold | 0.38 |
| 60 lb | 0.45 | 0.45 | 0.47 |
| 80 lb | 0.60 | 0.60 | 0.60 |
| 90 lb | 0.675, regional | 0.66 | — |
QUIKRETE Table 2 gives starting and maximum water in pints: 3 to 4.5 for 40 lb, 3.5 to 5.5 for 50 lb, 4 to 7 for 60 lb, 6 to 9 for 80 lb, 7 to 10 for 90 lb. The sheet multiplies the starting amount by the bag count and also shows gallons (8 pints to a gallon). Sakrete prints starting quarts only, and says more water may be needed but a soupy mix costs you strength. The data sheet also lists a placed unit weight around 140 lb per cubic foot. That figure includes the water. Do not divide your hole by 80 pounds using 140. You would be counting the water as if it came in the sack.
QUIKRETE’s sheet is for pours 2 inches thick or more, and it tells you to dig a post hole about three times the post diameter and about one-third the post height. Those are layout rules, not yields, so they are not buried in the bag count. Control joints on their slab method are about 1 inch deep, every 6 to 8 feet.
Mistakes that show up in the driveway
- Reading 55.56 bags as 55. You cannot buy the fraction, and the last half bag is the one that finishes the corner.
- Using one “Sakrete” yield for every red bag. High-Strength 60 lb and 5000 Plus 60 lb are not the same product.
- Treating a diameter as a radius, which quarters the concrete in a round hole.
- Ordering the slope wedge for steps that sit on flat ground.
- Forgetting that the yield is approximate. QUIKRETE says so. The allowance is how you say so with a number.
- Mixing past the maximum water because the mud was stiff. The strength numbers on the sheet are for a 2 to 3 inch slump, not for soup.
FAQ
How many 80 lb bags are in a cubic yard?
45, if the bag yields 0.60 cubic feet. QUIKRETE Concrete Mix No. 1101 and Sakrete High-Strength both publish 0.60 cubic feet for the 80 lb bag, and 27 ÷ 0.60 = 45. That count has no waste in it.
Should I add 10%?
Sakrete tells buyers to add 10% for safety. NRMCA CIP 8 and CIP 31 say to order 4% to 10% more than the volume from plan dimensions, for waste, over-excavation, and similar contingencies. This page starts at 10% and still prints the net volume, so you can slide it to 4% or to zero.
Why do some calculators say 62 bags for a 10 by 10 slab that only needs 56?
A 10 × 10 ft slab at 4 inches is 33.33 cubic feet, which is 56 bags at 0.60 cubic feet each. Sixty-two bags is that same pour plus 10%. A different error, using the weight of wet concrete instead of the bag yield, lands near 62 bags with no allowance at all. Those are not the same thing.
Are QUIKRETE and Sakrete 60 lb bags the same yield?
No. QUIKRETE No. 1101 and Sakrete High-Strength are both 0.45 cubic feet. Sakrete 5000 Plus is 0.47 cubic feet for the 60 lb bag, and its 50 lb bag is 0.38, not QUIKRETE’s 0.375. Pick the mix on the bag you are buying.
How much concrete is in porch steps?
On a flat grade, each step is a rectangle that reaches the ground, so the volumes stack: width × tread × rise × n(n+1)/2. Steps built into a slope are closer to a wedge: width × total rise × total run / 2. The wedge is smaller. Using it for a porch under-orders.
Sources
- QUIKRETE Concrete Mix data sheet, product 1101 (revised October 2022) — Yields for 40, 50, 60, 80, and 90 lb bags, Table 2 mixing water, 2 inch minimum thickness, and the post-hole rule of thumb.
- QUIKRETE concrete calculator — The brand calculator says its yields are approximate and do not allow for uneven substrate or waste.
- Sakrete High-Strength Concrete Mix, product page and TDS — 40 lb yields 0.30 cu ft, 60 lb yields 0.45, 80 lb yields 0.60. The TDS adds the 90 lb yield of 0.66 cu ft and starting water.
- Sakrete High-Strength TDS (PDF) — Yield and starting water by package size. Water is a starting reference; avoid a soupy mix.
- Sakrete 5000 Plus TDS (PDF) — 50 lb yields 0.38 cu ft, 60 lb yields 0.47, 80 lb yields 0.60. Starting water 2.1, 2.5, and 3.5 quarts.
- Sakrete Fast-Setting Concrete Mix — The 50 lb fast-setting bag is also published at 0.38 cu ft. Other fast-setting sizes are not assumed from 5000 Plus.
- Sakrete concrete calculators — Sakrete’s own tip: add 10% to estimates for safety.
- NRMCA CIP 8, Discrepancies in Yield — Order 4% to 10% over plan dimensions for waste, over-excavation, and other contingencies.
- NRMCA CIP 31, Ordering Ready Mixed Concrete — Repeats the 4% to 10% allowance and notes truck mixers commonly hold about 8 to 12 cubic yards.