What a bag of concrete actually gives you
A bag of pre-mixed concrete contains portland cement, sand and coarse aggregate already proportioned, so all you add is water. The specification behind it is ASTM C387/C387M, which covers packaged, dry, combined materials for concrete and mortar, and which is what the strength claim on the bag is tested against. What matters for a takeoff, though, is not the strength but the yield: the volume of finished concrete one bag makes.
The published yields are consistent across the major brands and they are the numbers this calculator uses:
- 80 lb bag → about 0.60 ft³
- 60 lb bag → about 0.45 ft³
- 50 lb bag → about 0.375 ft³
- 40 lb bag → about 0.30 ft³
Divide any of those into a pound and you get the same figure: 0.0075 cubic feet of concrete per pound of dry mix. That single constant is the whole calculator. It also means a cubic yard takes 27 ÷ 0.0075 = 3,600 lb of dry mix no matter which bag you buy — 45 eighty-pound bags, 60 sixty-pound bags or 90 forty-pound bags. Bag size is a decision about what you can lift and what your store stocks, not about how far the material goes.
The reason bags exist at all is that ready-mix has a floor. A batch plant will not send a truck for a wheelbarrow of concrete without a short-load surcharge, and for a handful of post holes or a mower pad the surcharge alone can exceed the material cost. Above roughly a cubic yard the arithmetic flips, and that is why this calculator flags the crossover point.
From geometry to bags, one division at a time
Every bag calculation is the same three moves: get the volume in cubic feet, inflate it for waste, divide by the yield and round up.
Rectangular pads are length × width × thickness with the thickness converted from inches to feet. A 6 ft × 3 ft pad at 4 inches is 18 × 0.3333 = 6.0 cubic feet.
Post holes are cylinders: π r² h, where r is half the auger diameter. Work in feet — a 10 inch auger gives r = 10 ÷ 24 = 0.4167 ft — and multiply by the depth of concrete, which is the hole depth less any gravel base you place under the post. Then subtract what the post itself occupies. A nominal 4×4 is actually 3.5 inches square, so it displaces (3.5 ÷ 12)² = 0.0851 ft² of cross-section for every foot of embedment. A 10 inch hole has a cross-section of π × 0.4167² = 0.5454 ft², so the post occupies 0.0851 ÷ 0.5454 = 16% of it — far too much to ignore across a whole fence line.
Known volumes from another takeoff go straight in; switch the unit selector to cubic yards or cubic metres and the conversion happens for you.
The rounding is up, always, and it is done once on the total rather than per hole. Take the six-hole fence in the worked example below: each hole needs 1.266 ft³ once the 10% allowance is on, which is 2.11 bags, so rounding hole by hole buys 3 bags each and 18 in total. Rounding the 7.60 ft³ total buys 13. The only case where per-item rounding is right is when you are mixing each hole separately and cannot carry mix between them — with a wheelbarrow or a mixer you can, and the total is what counts.
Worked example: six fence posts in 10 inch holes
You are setting six 4×4 fence posts. You are augering 10 inch holes 3 feet deep, placing 6 inches of gravel in the bottom of each for drainage, and filling the remaining 2 ft 6 in with concrete. You are buying 80 lb bags and want a 10% allowance.
- Depth of concrete. 3 ft − 0.5 ft of gravel = 2.5 ft.
- Hole radius in feet. 10 in ÷ 24 = 0.4167 ft.
- Gross volume of one hole. π × 0.4167² × 2.5 = 3.1416 × 0.17361 × 2.5 = 1.3635 ft³.
- What the post displaces. A 4×4 measures 3.5 in, so (3.5 ÷ 12)² × 2.5 = 0.08507 × 2.5 = 0.2127 ft³.
- Concrete in one hole. 1.3635 − 0.2127 = 1.1508 ft³.
- Six holes. 6 × 1.1508 = 6.905 ft³.
- Add 10% waste. 6.905 × 1.10 = 7.595 ft³.
- Divide by the 80 lb yield. 7.595 ÷ 0.60 = 12.66, rounded up to 13 bags.
- Weight and water. 13 × 80 = 1,040 lb of dry mix, needing roughly 13 × 6 pints = 78 pints, or 9.75 gallons of water.
Two checks are worth making. First, the volume is 6.905 ÷ 27 = 0.256 cubic yards, comfortably in bag territory. Second, notice what the post displacement saved: without it you would have needed 6 × 1.3635 × 1.10 ÷ 0.60 = 15 bags. Subtracting the post cut two bags off a six-post job, and on a hundred-post fence it cuts the order from 250 bags to 211.
Reading the result: bags, weight, water and the crossover
Look at the total dry weight before you look at the bag count. Thirteen 80 lb bags is 1,040 lb — a load that will squat most half-ton pickups on its own and that you will handle twice, once at the store and once at the hole. If the weight looks unreasonable for the job, the volume is probably wrong.
The mixing water figure is a planning number, not a specification. Packaged concrete mixes call for roughly 6 pints (0.75 US gallons) per 80 lb bag as a starting point, and this calculator scales that by weight. The bag itself gives a range, and you should mix to the wetness the product calls for rather than to a gallon count: too much water is the single most reliable way to make weak concrete, because strength falls as the water-to-cement ratio rises. Add most of the water, mix, then add the last of it a splash at a time.
The crossover to ready-mix sits near one cubic yard, which is 45 bags of 80 lb or 60 bags of 60 lb. Above that, the labour of opening, lifting and mixing bags dominates everything else, and the batch consistency of a truck is better than anything you will achieve in a barrow. Below about a third of a yard, bags almost always win. In between, get a delivered price including any short-load charge and compare it against the shelf price times the bag count — this calculator gives you both halves of that comparison.
One case runs against the general rule: fence and deck posts set dry. Some products are designed to be poured into the hole dry and wetted in place, which removes mixing entirely. That changes the labour calculation but not the volume, so the bag count on this page still applies.
Bag yields and coverage
| Bag size | Yield | Bags per yd³ | Dry weight per yd³ | Bags per ft³ | Bags for a 3×3 ft pad at 4 in |
|---|---|---|---|---|---|
| 40 lb | 0.30 ft³ | 90 | 3,600 lb | 3.33 | 10 |
| 50 lb | 0.375 ft³ | 72 | 3,600 lb | 2.67 | 8 |
| 60 lb | 0.45 ft³ | 60 | 3,600 lb | 2.22 | 7 |
| 80 lb | 0.60 ft³ | 45 | 3,600 lb | 1.67 | 5 |
The 3×3 ft pad at 4 in is 3.0 ft³ neat, rounded up with no waste allowance. The identical dry weight per cubic yard across every row is not a coincidence: all four yields work out to 0.0075 ft³ per pound.
Mistakes that send you back to the store
- Using the nominal post size instead of the actual. A 4×4 is 3.5 in and a 6×6 is 5.5 in. Using 4 and 6 overstates the displacement and understates the concrete.
- Filling the whole hole depth with concrete in the calculation but placing gravel in reality. Six inches of drainage stone under a 3 ft post removes a sixth of the volume.
- Rounding every hole up to a whole bag. Round the total, not the item, unless you genuinely cannot carry mix between holes.
- Buying to the exact count. Bags split, spill and sometimes arrive part-set from a wet pallet. Retailers take unopened bags back; a half-finished pour does not wait.
- Assuming a bigger bag goes further per pound. Every size yields 0.0075 ft³ per pound. Compare shelf prices per pound, not per bag.
- Mixing to a gallon count rather than to the bag's instructions. Extra water buys workability and costs strength.
- Mixing more than you can place. Working time is short, especially in heat. Mix in batches you can place and screed before the mix stiffens.
Mix type matters as much as bag count
Packaged mixes are not interchangeable. Standard concrete mix contains coarse aggregate and is meant for pours 2 inches thick and up. Sand mix and mortar mix contain no coarse aggregate and are for thin repairs and masonry bedding respectively — they will not develop the strength of a structural concrete in a footing. Fast-setting mixes are formulated for post setting and are typically poured dry into the hole and wetted. High-early-strength mixes reach a working strength sooner but cost more per bag. The yields above are for standard concrete mix; check the bag for any product that is not, because a different aggregate grading changes the volume a bag makes.
When bags stop being the right answer
Bags are the right choice for post holes, mower and condenser pads, splash blocks, small repairs and any pour where a truck cannot reach. Once the volume grows past roughly a cubic yard, three other options open up.
Ready-mix. Take the volume off properly first with the concrete slab calculator or the footing calculator, then get a delivered price. Ask specifically about the short-load charge, the standing time allowance and whether the truck can reach the forms.
Site-mixed from bulk materials. Buying cement, sand and stone separately and proportioning them yourself is cheaper per cubic yard than bags and gives you control of the mix. The proportions come from the concrete mix ratio calculator, and the aggregate quantities from the gravel tonnage calculator.
A different pour geometry. Round piers and Sonotube columns need the cylinder volume rather than the hole volume: use the column and Sonotube calculator. Cast steps come from the concrete stairs calculator. Deck piers should be sized for bearing with the deck footing size calculator before you take off any volume at all.
Two related quantities also come up on bag-sized jobs. If you are setting fence posts, the post spacing and picket count follow from the fence picket calculator. And if the job is a pad over a gravel base, size the base first — every inch of properly compacted stone you place is an inch of concrete you do not buy.
Key terms
- Yield
- The volume of finished concrete one bag produces when mixed with the specified amount of water. Published per bag size by the manufacturer; 0.60 ft³ for an 80 lb bag of standard concrete mix.
- Water-to-cement ratio
- The mass of mixing water divided by the mass of cement. It is the dominant control on strength: the more water beyond what the cement needs to hydrate, the weaker and more permeable the hardened concrete.
- Short load
- A ready-mix delivery below the plant's minimum, carrying a surcharge that often decides whether bags or a truck is cheaper on a small pour.
- Displacement
- The volume a post, sleeve or void occupies inside a hole, which is concrete you do not have to buy. Equal to the object's cross-sectional area times its embedded depth.