Concrete Pad Calculator
A pad is the smallest pour most people ever do, and the one where the size question comes first: how big does it need to be before you can work out how much concrete goes in it.
Concrete volume for equipment pads
The same pad at other thicknesses
| Thickness | Cubic yards | 80 lb bags | Good for |
|---|
How big does the pad need to be?
The pad should match the footprint of what sits on it, not the other way round. Equipment manufacturers publish a required pad size in the installation manual, and that number overrides anything on this page. What the table below gives you is the range those manuals usually land in, so you can sanity-check a plan before you dig.
| Pad for | Typical size | Thickness | What decides it |
|---|---|---|---|
| AC condenser | 30 × 30 to 36 × 36 in | 4 in | The unit's own base pan; 2 in clearance all round for airflow |
| Heat pump outdoor unit | 36 × 36 in | 4 in | Needs clearance for defrost water to drain away |
| Shed base | Shed footprint, no larger | 4 in | The floor frame, so the skirt covers the edge |
| Hot tub, 7 ft | 7 × 7 to 8 × 8 ft | 4 in, reinforced | The tub's spec sheet, plus a step-off edge |
| Trash or recycling cans | 24 × 24 to 30 × 30 in | 3 to 4 in | Wheelbase of the can, and where the lid swings |
| Standby generator | 48 × 36 in and up | 4 in | Manufacturer sets it; weight matters, not size |
| Pool equipment | 4 × 6 ft | 4 in | Room to stand in front of the filter and pump |
Ranges that residential installers work to. The installation manual for your unit is the authority, and some utilities specify a pad for rebate eligibility.
Four inches is enough until it isn't
A 4 inch slab on compacted gravel carries an AC condenser, a generator or a pair of trash cans without argument. The load spreads out and the soil takes it, which is what any slab on grade does. Going to 6 inches adds nothing except concrete you have to mix.
Two cases break that rule. A hot tub, because a filled 7 foot tub puts roughly 4,000 to 6,000 pounds on about 50 square feet and it does it permanently, not for an afternoon. And anything on fill, clay or a slope, where the pad is really a small structural slab and needs an engineer's eye rather than a thickness rule.
If the subgrade is the problem, fix the subgrade. Pouring a thicker pad over soft ground just makes a heavier thing to sink.
The base matters more than the pad
Every pad this calculator covers should sit on 2 to 4 inches of compacted crushed stone, not on bare dirt and not on topsoil. Stone drains, it does not heave when it freezes, and it gives you a flat surface to screed against. Bare soil under a pad holds water against the concrete and moves with the seasons.
Dig out the area about 6 inches larger than the pad on every side, fill with crushed stone, and compact it in two passes. That extra margin is what keeps the edges from breaking off when the ground around the pad settles.
Bags or a truck
A 36 by 36 inch pad at 4 inches is 3 cubic feet, about a ninth of a cubic yard, which is 6 bags of 80 lb mix. That is a morning with a wheelbarrow and a mixing tub. Ready-mix has a minimum charge that usually starts at one cubic yard plus a short-load fee, so for a single equipment pad the truck costs several times what the bags do.
The line sits around half a cubic yard. Below it, bags. Above it, price a short load, and remember that a truck cannot always reach a back yard, which decides the question before the money does.
One practical note on mixing: do not judge the water by how easy it is to stir. Add water to the range printed on the bag and no further. A pad mixed wet is a pad that dusts and scales within a couple of winters.
Precast and plastic pads: the option people forget
For an AC condenser or a heat pump, a precast concrete pad or a heavy plastic pad costs $40 to $150 and goes down in ten minutes on a levelled bed of stone. It is flat, it is already cured, and it can be lifted and reset if the unit has to move. For a single outdoor unit this is often the better answer than mixing bags, and no rule says a pad has to be poured in place.
Poured concrete wins when the pad is large, when it has to match a patio, when equipment weight is high, or when the pad needs to be anchored. Precast wins for one small unit and a Saturday you would rather not spend with a hoe.
Slope, and the one mistake that shows up as a rusted unit
A pad should be level for the equipment and pitched for the water. In practice that means flat across the footprint and a slight fall, about 1/8 inch per foot, away from the building. Water that ponds on a pad finds the bottom of the unit and stays there.
Keep the top of the pad at least 2 inches above the surrounding grade, and never below it. A pad that sits flush with the lawn becomes a puddle in spring, and the equipment sitting in it is the thing you replace first.
The measurement that catches people: pad sizes in equipment manuals are given in inches, and the concrete is ordered in cubic yards. A 36 by 36 by 4 inch pad is 0.11 cubic yards — a ninth of a yard, not a third. Multiply the three dimensions in inches, divide by 1,728 for cubic feet, then divide by 27 for cubic yards. Doing it in the wrong order is how a small pad turns into a bill for a full truck load.
Frequently asked questions
How much concrete do I need for an AC pad?
A 36 by 36 inch pad at 4 inches thick takes about 0.11 cubic yards, or 6 bags of 80 lb mix allowing 10 percent waste. Most condensers sit on 30 by 30 to 36 by 36 inch pads, so check the installation manual for the size the unit actually requires before mixing anything.
What size pad does an air conditioner need?
Usually 30 by 30 to 36 by 36 inches for a residential condenser, with 2 inches of clearance around the unit for airflow. The manufacturer's manual states the required footprint and some utilities specify a minimum pad for rebate eligibility, so that document decides it rather than a rule of thumb.
How thick should a concrete pad be?
4 inches on 2 to 4 inches of compacted crushed stone covers AC units, generators, trash cans and shed bases. A hot tub needs 4 inches with reinforcement because a filled tub puts 4,000 to 6,000 pounds on about 50 square feet. Anything on fill or clay needs a design, not a thicker slab.
Should I use a precast pad or pour one?
For a single AC condenser or heat pump, a precast or heavy plastic pad at $40 to $150 is faster, already cured and can be reset if the unit moves. Pour concrete when the pad is large, has to match adjacent paving, carries a hot tub, or needs anchors.
Do I need rebar in a concrete pad?
Not for an equipment pad at 4 inches on compacted stone. A hot tub pad and anything over roughly 100 square feet should have a light mesh or #3 bar for crack control. A pad on soft ground needs a better base before it needs steel.
How many bags of concrete for a 4 by 4 pad?
A 4 by 4 foot pad at 4 inches is 0.20 cubic yards, which is 9 bags of 80 lb mix before waste and 10 with a 10 percent allowance. At 6 inches the same pad takes 14 bags, or 15 with the allowance.
Can you pour a concrete pad over dirt?
You can, and it will crack and settle. Excavate 4 to 6 inches, fill with compacted crushed stone to within 4 inches of finish grade, and pour on that. The stone is what keeps the pad flat through freeze and thaw, and it costs less than the concrete it saves.
Disclaimer: This calculator estimates volume from the dimensions you enter and a standard bag yield. It does not check the pad size against your equipment. Pad dimensions, thickness and anchoring are set by the manufacturer's installation manual and by local code, and a pad carrying a hot tub or sitting on fill should be reviewed by a licensed engineer.
Citing this page? FigureMill, “Concrete Pad Calculator” (last reviewed October 31, 2026). Concrete volume in cubic yards and bag counts for equipment pads, with typical pad dimensions and thickness by equipment type and a bag-versus-ready-mix comparison. https://figuremill.com/materials/concrete-pad-calculator
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