Ready mixed concrete delivered by truck is the right answer for a driveway or a house slab. For a run of fence post footings, a shed base, a set of steps or a garden wall foundation, ordering a truck is impractical and expensive, and the minimum load is usually far more than the job needs. That leaves mixing on site, where the two things that decide whether the finished pour is sound are getting the proportions right and getting the water right.
Both are simpler than the surrounding folklore suggests. Concrete proportioning by volume has been done the same way for a century, needs no scales, and produces reliable results provided you measure consistently and resist the temptation to add water to make the mix easier to move.
Working Out How Much You Need
Start with volume, because ordering short halfway through a pour causes a cold joint that becomes a permanent weakness.
For a slab, multiply length by width by thickness with every figure in feet, which gives cubic feet. Divide by 27 for cubic yards. A pad 8 feet by 6 feet at 4 inches thick works out at 8 times 6 times 0.333, or 16 cubic feet, which is roughly 0.6 of a cubic yard.
For post holes, treat each as a cylinder: radius squared times pi times depth. A 10 inch diameter hole 30 inches deep is about 1.4 cubic feet, so twelve fence posts need around 17 cubic feet before you allow for the volume the post itself displaces.
Add 10 percent for spillage, over excavation and the material that stays stuck in the mixer. Bagged premix yields are printed by the manufacturer, and the common figures are about 0.6 cubic feet from an 80 pound bag, 0.45 from a 60 pound bag and 0.3 from a 40 pound bag. Those numbers make the choice between bags and raw ingredients obvious quickly, because past roughly 20 cubic feet the bag count becomes absurd and buying cement, sand and aggregate separately costs far less.
Mix Proportions by Volume
The classic general purpose mix is 1 part cement, 2 parts sharp sand and 3 parts coarse aggregate, measured by volume with the same bucket throughout. This lands in the region of 3,000 psi once cured and suits slabs, footings and most domestic structural work.
For non structural work such as fence post backfill or a light garden path, 1 part cement to 2 parts sand to 4 parts aggregate is adequate and uses less cement. For anything carrying real load, do not lean that way. The cost difference across a small pour is trivial and the strength difference is not.
The measuring discipline matters more than the exact ratio. Use one bucket for everything, fill it the same way each time, and count out loud. Mixed batches that differ from each other produce a slab with variable strength, which is worse than a slab that is uniformly slightly weak.
One point that catches people out is shrinkage. Dry loose ingredients pack down when mixed and wetted, so the finished volume is roughly two thirds of what went in. To place 15 cubic feet of concrete you need to measure out around 22 cubic feet of dry material.
Water, and Why Less Is Better
Water is where small pours go wrong. Cement needs a modest amount of water to hydrate, and every additional litre beyond that dilutes the paste and reduces final strength. A water to cement ratio around 0.5 by weight is the usual target, which for a 25 kilogram bag of cement means roughly 12 litres of water across the whole batch.
The practical test is the mix itself. Correctly proportioned concrete looks stiff and slightly reluctant. Drag a shovel through it and the furrow should hold its shape with the sides standing rather than slumping closed. If the mix flows easily, it is too wet, and the strength loss is significant rather than marginal.
Add water in stages and hold some back. It is easy to add more and impossible to remove it without adding dry material, which then throws the proportions off. Aggregate that has been standing in rain already carries water, so start drier than you expect to finish.
Choosing How to Mix It
Hand mixing in a barrow or on a board is fine up to about 3 or 4 cubic feet. Past that, hand mixing produces tired arms and, more importantly, inconsistent batches, because fatigue leads to shortcuts.
A paddle mixer driven by a heavy duty mixing drill suits stiff mortar, thinset, render and modest concrete batches in a bucket or tub. Corded units such as the Ridgid R7122 9 amp mixer are built for this, with the low speed and high torque a loaded paddle demands. Cordless options such as the DEWALT FLEXVOLT DCD130T1 add an electronic clutch, which matters because a paddle that snags in a stiff mix can otherwise wrench the tool out of your hands. The concrete mixers category covers both approaches.
Beyond about 10 cubic feet in a session, a drum mixer hired for the day is the sensible route. It mixes more uniformly than any paddle and lets one person keep pace with a pour.
If the job starts with breaking out old concrete, that is a separate tool question, and a rotary hammer such as the DEWALT DCH133B will move far more material than a hammer drill attempting the same work.
Key Takeaways
- Calculate volume first, then add 10 percent for waste and losses.
- 1 cement to 2 sand to 3 aggregate by volume suits most structural domestic pours.
- Dry ingredients shrink to roughly two thirds of their loose volume once mixed.
- Excess water is the main cause of weak concrete; aim for a stiff, furrow holding mix.
- Use one bucket for every measurement and keep batches identical.
Frequently Asked Questions
How many bags of premix do I need for a post hole?
A typical 10 inch hole at 30 inch depth takes roughly 1.4 cubic feet, so about two and a half 80 pound bags before allowing for the post. Most people budget two to three bags per post depending on hole diameter.
Can I mix concrete in a wheelbarrow?
Yes, for small quantities. Turn the material dry first until the colour is uniform, form a crater, add part of the water and fold inward. Wheelbarrow mixing becomes unreliable past three or four cubic feet because consistency slips as you tire.
What is the difference between concrete and mortar mix?
Concrete contains coarse aggregate and is meant to carry structural load. Mortar is cement and sand only, formulated to bond masonry units and stay workable on a trowel. Mortar used as concrete will not reach the same compressive strength.
How long before concrete can take load?
Foot traffic is generally acceptable after 24 to 48 hours in mild conditions. Concrete reaches most of its design strength at 28 days, and keeping the surface damp for the first several days after the pour makes a measurable difference to the final result.
