Concrete Mixer Calculator

1. Enter the dimensions of the pour and a 5 to 10 percent waste allowance, as you would for any concrete job.
2. Pick the bag size you are mixing.
3. Enter your mixer drum size in cubic feet and the usable fill, about 50 to 60 percent of the drum.
4. Read the result: the number of batches, how many bags go in each, and the total bags for the job.

Here is what each result line means:

A concrete mixer calculator works out how many batches your mixer needs to finish a pour, so you can judge whether the job is realistic to mix by hand or better ordered ready-mix. It turns a volume into a batch count, the number that actually governs the time and effort.

The key fact it captures is that a mixer only mixes part of its drum at once. A 3.5 cubic foot drum typically mixes about 1.75 to 2 cubic feet of concrete per batch, roughly half the drum, because the material needs room to tumble. The tool sizes the pour, converts it to bags, works out how many whole bags fit per batch, and divides. To size the mix and materials that go into each batch, pair it with the concrete mix calculator.

First the tool finds the pour volume and the total bags, the same way a bag calculator does. Then it works out how many whole bags fit in one batch: the drum size times the usable fill gives the batch capacity in cubic feet, divided by the bag yield and rounded down to whole bags.

1. Size the pour volume in cubic feet and add the waste allowance.
2. Divide by the bag yield and round up for the total bags.
3. Multiply the drum size by the fill to get the batch capacity, then divide by the bag yield and round down for whole bags per batch.
4. Divide total bags by bags per batch and round up for the number of batches.

Suppose you are pouring a slab 10 feet by 10 feet at 4 inches, with 10 percent waste, 80 pound bags and a 3.5 cubic foot mixer filled to 50 percent.

Calculation: the slab needs about 62 eighty-pound bags. The batch capacity is 3.5 x 0.50 = 1.75 cu ft, and at 0.60 cubic feet a bag that is 2 whole bags per batch. So 62 / 2 = 31 batches.

The lesson: 31 batches is a full day of mixing. A number that high is the signal to rent a bigger mixer or price a ready-mix delivery instead.

Drum Size and Fill

The batch capacity is the drum size times the usable fill, and only about half the drum mixes well. A bigger drum, or a mixer that lists a higher mixing capacity, drops the batch count fast.

Bag Size

Larger bags hold more per bag, so fewer fit per batch but each batch places more concrete. The batch count follows the bags that physically fit, not the drum volume alone.

Pour Volume

Batches rise in step with the volume. A small pad is a few batches; a driveway is dozens, which is why volume quickly decides mixer versus ready-mix.

Overfilling

Loading past the usable fill bogs the motor and spills mix. The fill figure keeps each batch to what the drum can actually turn over.

The batch count is really a labour estimate. A handful of batches is an afternoon; dozens is a hard day, and the point where a ready-mix delivery wins on time, if not always on material cost.

For a large job, size a delivery with the concrete truck calculator, and get the water right in each batch with the water cement ratio calculator.

Planning a DIY Pour

Before you start, see how many batches a job will take so you can decide whether your mixer and your day are enough.

Choosing or Renting a Mixer

Compare drum sizes by the batch count they give for your pour, and rent up a size if the number is daunting.

Deciding Mixer vs Ready-mix

Use the batch count as the tipping point: once it runs into the dozens, a delivery usually saves the day.

1. Using the Drum Size as the Batch Size

A 3.5 cubic foot drum does not mix 3.5 cubic feet. Only about half turns over properly, so use the fill figure or the mixer’s listed mixing capacity.

2. Overfilling to Save Batches

Cramming the drum bogs the motor and throws mix out. It does not actually mix faster, and it wastes material.

3. Forgetting Waste

Leaving out the waste allowance under-counts bags and batches, so you run short mid-pour. Keep 5 to 10 percent.

4. Ignoring the Time Cost

Each batch is several minutes of loading, mixing and emptying. Thirty batches is a day, not an hour, which the bag total alone hides.

5. Mixing a Pour That Should Be Ready-mix

Past a couple of cubic yards the batch count makes hand mixing impractical. Check the batches before you commit to a mixer.

The arithmetic and the bag yields are exact; the result depends on the drum size and fill you enter, which vary by mixer model.

Only about half the drum volume at a time, because the mix needs room to tumble. A 3.5 cubic foot mixer mixes roughly 1.75 to 2 cubic feet per batch, which is about two 80 pound bags or three 60 pound bags. Overfilling bogs the motor.

A cubic yard is about 45 eighty-pound bags, and a 3.5 cubic foot mixer takes about two whole bags per batch, so roughly 23 batches. A larger mixer that holds more bags per batch cuts that down. Enter your drum size and bag size above for your pour.

About 1.75 to 2 cubic feet of mixed concrete per batch, not the full 3.5 cubic feet, because the drum must have room to turn the mix over. That is the usable mixing capacity, which is what the batch count is based on.

No. Filling past about half to sixty percent of the drum stops it mixing properly, strains the motor and spills material. Keep each batch to the usable fill, or to the mixing capacity your mixer lists.

Loading, mixing and emptying a batch takes several minutes, plus moving the concrete. That is why a batch count in the dozens means a full day of work, and why it is the real measure of whether to mix by hand.

Once the batch count runs into the dozens. A few batches is an easy job; twenty or thirty is a hard day. At that point a larger mixer, or a ready-mix delivery, usually saves more time than it costs.

Not necessarily. Bigger bags hold more concrete, so fewer fit per batch but each batch places more. The batch count depends on how many whole bags fit the drum, which the tool works out from your bag size and fill.

For posts, fast-setting mix is usually poured dry into the hole, not mixed in a drum, because of its short set time. A standard concrete mix is what you batch in a mixer. Use this tool for standard mixes.

No. The calculation runs entirely in your browser and nothing you enter is stored or sent anywhere.

• Concrete Decor, choosing a portable concrete mixer (Concrete Decor; mixing capacity and choosing a mixer).
• QUIKRETE Concrete Mix 1101 data sheet (QUIKRETE; bag yields 0.60, 0.45 and 0.30 cu ft).
• National Ready Mixed Concrete Association (NRMCA; ready-mixed concrete for larger pours).