Concrete & Mortar Calculator
Add your pours - slabs, post holes, and strip footings - and instantly see how many bags to buy and how much water to mix. Build a project list with multiple pour types. Nothing uploaded.
Learn more: concrete volume, bags and mix water
How the volume is worked out
A slab or a strip footing is length × width × depth in metres, which gives cubic metres straight off. A post hole is treated as a cylinder, π × radius² × depth, multiplied by however many holes you enter.
The tool measures each pour on its own, then adds them up. The default project shows it: a 4 × 3 m slab at 0.10 m thick is 1.2 m³, six 300 mm holes dug 900 mm deep add 0.382 m³, and the total reads 1.582 m³.
Bag counts round up once, at the end
Every bag size has a fixed yield of mixed concrete: 0.009 m³ for a 20 kg bag, 0.011 m³ for 25 kg, and 0.008, 0.012 and 0.016 m³ for the 40, 60 and 80 lb bags. The tool divides your total volume by that yield and rounds up to a whole bag.
The rounding lands once on the project total, not on each pour, so splitting a slab into two entries gives the same count as entering it whole. That 1.2 m³ slab comes to 110 bags at 25 kg, or 134 at 20 kg.
The tool adds nothing for waste. Spillage and an uneven subgrade both eat into a pour, so it is normal to buy 5 to 10% over the calculated figure rather than exactly what the numbers say.
Post holes lean the other way. The cylinder figure assumes an empty hole filled to the brim, and a 100 mm square post standing in a 300 mm hole takes up about 14% of it, so that pour already has slack in it.
Mix water follows the grade, not the bag
Only two things feed the water figure: the mix you pick and the total volume. C20 uses 155 litres per cubic metre, C25 uses 165 and C30 uses 175, so the 1.582 m³ default needs 245 litres as C20 and 277 as C30.
Higher grades carry more cement, so the litres per cubic metre climb even though structural concrete is mixed drier against its cement content. Extra water makes a pour easier to place and finish, and it weakens the concrete once it sets.
Mortar is wetter again: 270 litres per cubic metre at 1:3 cement to sand, 255 at 1:4. Those figures are not scaled from the concrete grades, since mortar has no coarse aggregate taking up space.
Bag size plays no part in any of this, so treat the number as a batch total and not a dose per bag. Bagged ready-mix already holds its own cement-to-aggregate ratio, so mix to the water instruction printed on the bag and use this figure as a check on how much water to have ready.
What the grade numbers mean
The C number is the 28-day compressive strength in megapascals, so C20 reaches 20 MPa and C30 reaches 30. C20 covers garden slabs and fence posts. C25 is the usual pick for driveways and anything taking vehicle weight, and C30 is for structural work.
Most of that strength arrives early: roughly two thirds by day 7 and close to 90% by day 14. Cold weather slows the whole thing down and can leave a pour short of its design strength.
FAQ
Why doesn't the bag count change when I switch to a mortar mix?
The mix selector only drives the water figure. Bags come from total volume and bag size alone, so moving from C20 to a 1:3 mortar changes the litres and leaves the bag count exactly where it was.
How much concrete does one fence post hole take?
A 300 mm hole dug 900 mm deep holds about 0.064 m³. Six of them come to 0.382 m³, which is 35 bags at 25 kg if the posts are the whole job. Use the hole count field rather than adding a separate pour per post.
At what point is a ready-mix truck worth it over bags?
The default project here is 1.582 m³, or 144 bags at 25 kg, which is a long day of mixing by hand. Somewhere around 2 m³ most people switch to a delivery, though suppliers set their own minimum load so it is worth a phone call before deciding.