Choosing a concrete mixer that fits the job
Concrete mixers are sold by drum volume – the total capacity of the drum, such as 125 L, 160 L or 4 cubic feet. You can’t fill a drum to the brim: it must tilt to mix and the concrete needs room to tumble. In practice a mixer produces roughly 55–65% of its drum volume as finished concrete per batch. A small 63 L mixer turns out about one 80 lb bag of premix per batch; a 9 cu ft tow-behind mixer handles six to eight.
The right size is the one that lets you place the whole pour within the working time of the concrete with the people you have. Each batch takes about 5–8 minutes to load, mix and discharge. For a slab that must be poured in one go, too many small batches means the first part starts setting before the last part is placed, leaving visible cold joints.
How volume is calculated
Slab: length × width × thickness.
Footing: length × width × depth.
Post holes: π × (diameter ÷ 2)² × depth × number of holes.
One cubic yard is 27 cubic feet (0.765 m³). The calculator adds a waste allowance because forms bulge, ground is uneven and some concrete always stays in the drum and wheelbarrow.
Bags or site-mixed?
An 80 lb (36 kg) bag of premix yields about 0.6 ft³ (17 L) of concrete. Bagged premix is convenient for small jobs. For larger volumes, buying cement, sand and gravel separately is cheaper: dry materials shrink by about a third when water is added, so the calculator multiplies the wet volume by 1.54 to find the dry materials.
Common mixer sizes
| Drum size | Concrete per batch | 80 lb bags per batch | Typical power | Good for |
|---|---|---|---|---|
| 63 L (2.2 ft³) | ≈ 35 L (1.3 ft³) | 2 | 120/230 V, ~250 W | Posts, small repairs, mortar |
| 125–140 L (4–5 ft³) | ≈ 75–85 L (2.7–3 ft³) | 4–5 | 120/230 V, ~550 W | Patios, footings, DIY projects |
| 160–180 L (6 ft³) | ≈ 95–110 L (3.4–3.8 ft³) | 5–6 | 230 V or gas engine | Larger slabs, regular use |
| 9 ft³ (255 L) towable | ≈ 150 L (5.4 ft³) | 8–9 | Gas engine | Contractors, slabs up to ~3 yd³ |
When to order ready-mix instead
Above about 1.5–2 cubic yards (1.2–1.5 m³) for a single continuous pour, ready-mixed concrete delivered by truck is usually faster, more consistent and often cheaper than mixing by hand. Many suppliers deliver “short loads” of one yard or less for an extra fee. A mixer still makes sense for jobs spread over several days, remote sites a truck can’t reach, or when you need small amounts often.
Mixing tips
- Add about two-thirds of the water first, then the gravel, cement and sand, then the rest of the water little by little.
- Mix for at least 3 minutes after the last material is added, until the color and consistency are even.
- Don’t add extra water to make placing easier: every extra liter weakens the concrete. Aim for a stiff, oatmeal-like mix.
- Rinse the drum with water and some gravel at breaks and immediately after the last batch.
- Wear gloves, eye protection and long sleeves: wet cement is alkaline and can burn skin.