If you’re standing in the aisle wondering how many cement bags for a slab you actually need, here’s the straight answer: a 10×10 foot slab at 4 inches thick requires about 56 80‑lb bags or 74 60‑lb bags of premixed concrete, plus 5–10% for waste. A 20×20 slab at the same thickness needs roughly 224 80‑lb bags or 296 60‑lb bags. Those numbers assume you’re buying what most folks call “cement bags” but are really bagged concrete mix. If you’d rather skip the arithmetic, our Cement Bag Calculator does it instantly, but understanding the math saves you from costly shortages on pour day.
The “Cement” vs. Concrete Bag Confusion
First, let’s kill the terminology mix‑up that trips up every beginner. When a homeowner asks for “cement bags,” they almost always mean the gray bags of ready‑to‑use concrete mix from a hardware store. Pure cement is just the binder; you can’t pour a slab with Portland cement alone because it lacks sand, gravel, and the proper water ratio.
I learned this the hard way on my first shed pad. I walked into a lumberyard and asked for “ten bags of cement.” The clerk handed me ten 94‑lb sacks of pure Portland. I got home, mixed one with water, and ended up with a chalky, cracked mess that crumbled under a wheelbarrow. That mistake cost me a weekend and $40 in wasted material.
So for the rest of this guide, treat “cement bag” as shorthand for an 80‑lb, 60‑lb, or metric 20‑kg bag of premixed concrete. The yields we discuss are for those products, not raw cement. Most DIY guides use the words interchangeably, but in the field they are not the same. Concrete is the structural material; cement is one ingredient.
I keep a photo on my phone of that first ruined shed pad to show apprentices the difference. The pure Portland cement I bought formed a surface crust that cracked within hours because it shrank without aggregate to lock it. That visual is worth more than any disclaimer. One non‑obvious insight: bagged “concrete” from different brands can have different aggregate sizes, which changes yield by a few percent. Always read the yield printed on the bag, not just the weight.
The Universal Slab Bag Formula (Step‑by‑Step Manual Math)
You don’t need an engineering degree, just a tape measure and the equation I use on every job:
Bags needed = (Length × Width × Thickness) ÷ Bag Yield
Measure length and width in feet. Convert thickness from inches to feet by dividing by 12. The result is volume in cubic feet. Then divide by the yield of one bag in cubic feet.
According to Quikrete’s product specifications, a standard 80‑lb bag yields about 0.594 cubic feet (0.022 cubic yards), and a 60‑lb bag yields roughly 0.45 cubic feet (0.0167 cubic yards). Metric 20‑kg bags typically yield around 0.009 cubic meters, which is close to 0.318 cubic feet.
To make this practical, memorize the bags‑per‑yard constants: 45 80‑lb bags or 60 60‑lb bags equal one cubic yard of finished concrete. That’s easier than dividing fractions on a muddy site. If you know your slab is 2 yards, you immediately know 90 or 120 bags before waste.
Walking Through a Real Calculation
Take the 10×10 slab at 4 inches. Length 10 ft, width 10 ft, thickness 4÷12 = 0.333 ft. Volume = 10 × 10 × 0.333 = 33.3 cubic feet. Divide by 0.594 (80‑lb yield) = 56.1 bags. Round up to 57, then add waste.
For the 60‑lb bag, 33.3 ÷ 0.45 = 74 bags exactly. This matches the cheat sheet later. The thing nobody tells you about these yields: they assume a perfectly flat, non‑absorbent subbase. In reality, uneven dirt drinks up concrete, so your effective yield drops by a few percent.
When I poured a barn floor on clay, I measured everything twice, yet the ground sucked moisture so fast that the first 20 bags settled half an inch lower than forms. I had to order 8 extra bags mid‑pour—a costly lesson in subbase prep.
Handling Odd Shapes and Slopes
If your slab isn’t rectangular, break it into squares or use the average width. For a gently sloped slab, calculate the average thickness (thick end + thin end ÷ 2). I once poured a wheelchair ramp that averaged 5.5 inches instead of a flat 4; ignoring the slope would have left me 30 bags short.
For circular slabs, use π × radius² × thickness. A 12‑foot diameter patio at 4 inches is about 113 sq ft, needing 63 80‑lb bags. Most people don’t realize round slabs waste more at the form edge if you’re using square bags of mix? No—waste is same, but cutting expansion joints matters.
Slab Bag Cheat Sheet: Common Sizes at 4″ and 6″ Thickness
Below is the cheat sheet I keep in my truck. It covers the two most‑asked scenarios and answers the classic search: how many 80 pound bags of concrete do I need for a 10×10 slab? The tables also show 60‑lb counts and metric conversions for a 20‑kg bag (using 0.318 cu ft yield).
10×10 Slab Bag Counts
- 4‑inch thickness: 56 × 80‑lb bags | 74 × 60‑lb bags | ~105 × 20‑kg bags
- 6‑inch thickness: 84 × 80‑lb bags | 111 × 60‑lb bags | ~157 × 20‑kg bags
Notice the 6‑inch slab isn’t 50% more bags than 4‑inch; it’s exactly 50% more volume, so the count scales linearly. That answers the 10×10 question plainly: at 4″ you need 56 80‑lb bags before waste. Add 10% and you’re at 62 bags.
I printed this table on a laminated card after a job where my phone died and I couldn’t access any online calculator. Field conditions—rain, dust, dead batteries—are why a physical cheat sheet beats a web app.
20×20 Slab Bag Counts
- 4‑inch thickness: 224 × 80‑lb bags | 296 × 60‑lb bags | ~420 × 20‑kg bags
- 6‑inch thickness: 336 × 80‑lb bags | 444 × 60‑lb bags | ~630 × 20‑kg bags
If you’re asking how many bags of cement do I need for a 20×20 slab, the short answer is 224 80‑lb bags at 4 inches. But most homeowners shouldn’t hand‑mix that many bags; we’ll cover why in the cost section.
One more nuance: 20×20 at 6 inches is a heavy‑load slab for a workshop. You’ll likely need 3500–4000 psi mix, which may have different yield. Check the bag; don’t assume the 336 count holds if you switch to high‑strength.
How Much Area Will a 60 lb Bag of Quikrete Cover?
This pops up constantly, so let’s nail it. A 60‑lb bag yields 0.45 cubic feet. At 4‑inch (0.333 ft) thickness, it covers 0.45 ÷ 0.333 = 1.35 square feet. At 2 inches, it covers 2.7 sq ft; at 6 inches, only 0.9 sq ft. So a single 60‑lb bag of Quikrete is enough for a patch roughly 1.1 ft by 1.2 ft at slab depth.
In my experience, that coverage claim printed on the bag is optimistic if you float the surface with a squeegee; the slight extra troweling can consume another 5% material. Plan for the lower end.
Waste, Overage, and the Mistakes That Inflate Your Bag Count
Always order 5–10% extra. I default to 10% on open‑grade dirt, 5% on a prepared gravel base with forms. The most common error is ignoring spillage and the concrete that hardens on your mixing tools.
When I first tried a large pour with borrowed wheelbarrows, I lost a full bag’s worth to spills on the grass because the barrows had cracked pans. Another hidden loss: evaporation on hot days. A slab poured at 90°F can lose surface moisture, requiring a slightly richer mix or extra water that reduces yield.
Rule of thumb: if you can’t explain where every bag’s concrete went, you didn’t order enough overage.
Also, don’t confuse “bag count” with “cement count” if you’re using a mixer and adding separate cement, sand, gravel. That’s a different formula and almost never worth it for a slab under 2 yards.
Leftover bags are a storage problem. Cement in opened bags absorbs humidity and hardens into a brick within two weeks in my humid climate. Buy sealed bags, and return unopened extras—most stores allow returns within 90 days.
The thing nobody tells you about overage: if you order exactly the math, you will run short. Concrete is the one material where a 2% shortage means a cold joint and a weakened slab. I’ve seen DIYers try to stretch the last bag with extra water; that weakens psi by up to 30%.
Is It Cheaper to Mix Your Own Concrete or Buy Bags? (Real Cost Breakdown)
The question “is it cheaper to mix your own concrete or buy bags?” depends entirely on volume and your labor value. Let’s break it down with 2024 retail numbers from big‑box stores and local ready‑mix quotes I gathered in three states.
Bagged Concrete Economics
An 80‑lb bag runs $3.50–$5.00. Since 45 bags make a cubic yard, material cost is $157–$225 per yard. A 60‑lb bag costs $2.80–$4.00; 60 bags per yard means $168–$240 per yard. That excludes the water, mixer rental ($50/day), and your time.
Ready‑Mix Truck Economics
A ready‑mix truck charges $125–$150 per cubic yard plus a $75–$100 short‑load fee if under 5 yards. For a 20×20 slab at 4 inches, volume is about 4.94 yards. Truck cost ≈ $700–$850 delivered. Bagging the same slab would be 224 × $4 = $896 just for 80‑lb bags, before mixer and sweat.
| Method | Cost per Yard | Labor Hours (1 person) | Best For |
|---|---|---|---|
| 80‑lb Bags | $157–$225 | 8–10 per yard | Small <1 yd, no truck access |
| 60‑lb Bags | $168–$240 | 10–12 per yard | Medium 1–2 yd, lighter lifting |
| Ready‑Mix Truck | $125–$150 + fees | 1–2 per yard | Large 3+ yd, flat access |
The Labor and Time Trade‑Off
- Small slabs (under 1 yard): Bags win because truck minimums kill the deal.
- Medium slabs (1–3 yards): Bags possible but physically brutal; a mixer and 2 people can do 1 yard in 4 hours.
- Large slabs (3+ yards): Truck is cheaper and far less injury‑risk. Mixing 200+ bags by hand is a recipe for a trip to the ER.
The honest limitation: if you have no truck access (e.g., fenced backyard), bags may be your only option despite cost. I’ve hauled bags through a 30‑inch gate because the concrete truck couldn’t reach—sometimes convenience overrides price.
Another factor: cleanup. Bag mixing leaves dust and empty sacks; truck pour leaves a washout bucket. Neither is free, but bags generate more landfill waste.
Metric Considerations and Global Bag Sizes
Outside the US, the 20‑kg bag reigns. It yields about 0.009 m³. For a 3‑meter by 3‑meter slab (roughly 10×10 ft) at 100 mm thickness, volume = 0.9 m³. Divide by 0.009 = 100 bags. That aligns with the ~105 bags we listed earlier allowing waste.
If your local store sells 25‑kg or 40‑lb bags, recalculate using the printed yield on the sack. Never trust a uniform global number; aggregate size changes density. In the UK, a “cement bag” often means 25 kg of Portland, but “postcrete” or “concrete mix” bags are the slab solution.
I worked on a cabin in Portugal where the bags were 35 kg and yielded 0.015 m³. My cheat sheet failed until I wrote the local yield on the back. Always localize the formula.
Advanced Edge Cases: When the Formula Needs Tweaks
Most slab calculators assume a void‑free pour. But if you add rebar or wire mesh, the displacement is negligible (<1%). A gravel base with voids, however, can swallow 5% more concrete before the forms fill.
High‑strength mixes (e.g., 5000 psi) often have slightly lower yield per bag because of reduced air entrainment. And if you’re pouring over a vapor barrier, you’ll waste less to absorption—so you can trim overage to 5%.
Another edge case: curved or octagonal slabs. Use the inscribed rectangle for a conservative estimate, then add 10% because cutting and fitting wastes material at edges.
Temperature matters. At freezing temps, you may need additives that don’t change volume but slow pour, increasing spill risk. In extreme heat, wind can crust the surface before you screed, causing you to over‑trowel and use more mix.
Finally, if you mistakenly buy pure cement (as I did), the slab formula does not apply. You’d need 1 part cement, 2 parts sand, 3 parts gravel by volume, which yields roughly 4 parts concrete—but that’s a separate workflow I avoid for slabs under 5 yards.
My Go‑To Workflow for Ordering Slab Bags
After a dozen slabs, here’s the checklist I run before buying:
- Measure length, width, and intended thickness twice.
- Compute volume with the universal formula above.
- Pick bag size based on who’s lifting (80‑lb is a back‑breaker; 60‑lb is manageable).
- Add 10% waste for dirt base, 5% for gravel + vapor barrier.
- Compare bag total cost vs ready‑mix using the yardage.
- Confirm access for delivery; if none, bags it is.
Core equation to tape to your toolbox: Bags = (L × W × thickness in ft) ÷ bag yield.
Following this, a 10×10 slab at 4 inches becomes a predictable 62 bags (with 10% waste) rather than a panic buy at midnight. That’s the difference between a weekend project and a failed inspection.
If you want a digital fallback, our Cement Bag Calculator mirrors this math and stores your last inputs. But the laminated card stays in my truck because cell service fails at remote build sites.
The last insight: concrete is heavy, and bags are awkward. Schedule pickup with a pallet jack or have them delivered to the driveway. I once hand‑carried 80 bags from a curb to a backyard—my chiropractor appreciated the business, I did not.