What is an AC pad and what does it do?
An AC pad is the base that supports the outdoor condensing unit, and it does four things: it keeps the unit level, lifts it a few inches off wet ground, spreads the unit's weight so it does not sink into soft soil, and dampens the vibration that would otherwise travel into the ground and the refrigerant lines. Without a solid pad, a condenser slowly tilts, and a tilted unit is hard on the compressor and the copper lineset.
The pad sits under the outdoor unit, which is the condenser that holds the compressor, coil, and fan. Two things connect that unit to your house: the insulated copper refrigerant lineset and the electrical whip running from the disconnect box on the wall. Both are fairly rigid, so when the pad shifts, those connections take the strain. That is why keeping the pad flat matters more than it looks.
Pads come in two common forms. A poured concrete slab is the traditional, heaviest, most permanent option. A composite pad (a molded plastic or rubberized panel, often a brand like DiversiTech) is lighter, cheaper, and will not crack or heave in a freeze. Both work; the choice comes down to your soil, your budget, and whether the unit is a ground install or on a roof.
What should you put under an air conditioner?
Put a purpose-made pad under an air conditioner, not bare dirt or a loose pile of gravel. The two right answers are a poured concrete slab or a molded composite condenser pad set on a compacted gravel base. Both give you a flat, load-bearing surface that drains and stays put. Gravel alone spreads and washes out, and bare soil turns to mud and lets the unit settle unevenly.
Composite pads have taken over most new installs for good reasons. They weigh a few pounds instead of a hundred, they do not crack when the ground freezes and thaws, and they cost a fraction of a concrete pour. A common setup is two to three inches of compacted gravel or paver base, leveled, with the composite pad set on top. That gravel layer is the part that actually keeps things level, so do not skip it.
Concrete still makes sense in specific cases: very heavy units, sites with pooling water, or anywhere you want a truly permanent base. Pavers can work as a budget middle ground if they are large, thick, and set on a proper compacted base rather than dropped on the lawn. Whatever you choose, keep at least two feet of clearance around the unit so it can breathe, the same airflow rule that keeps the condenser from overheating.
What size and thickness does an AC pad need to be?
An AC pad should be a couple of inches larger than the unit's footprint on every side, and thick enough to carry the weight without flexing. The most common residential pad sizes are 32 by 32, 36 by 36, and 38 by 38 inches, with 30 by 30 used for smaller units. Measure the base of your condenser and add roughly two inches per side so the feet sit fully on the pad, never hanging over an edge.
For thickness, a composite pad is usually 2 to 3 inches thick, and a poured concrete slab is typically 3 to 4 inches. The composite pad relies on the compacted base under it for support, while the concrete carries the load itself. Either way, the goal is a rigid surface that will not sag under a unit that can weigh 100 to 200 pounds full of refrigerant.
Size matters for stability, not just fit. A pad that barely matches the footprint has no margin, so any small shift leaves a foot hanging off the edge. Going one size up is cheap insurance. If you are shopping for a new system and want to know the capacity your home actually needs before the installer sizes the pad, run the numbers with our BTU calculator.
How much does an AC pad cost?
A composite AC pad costs $30 to $90 for the pad itself in the common 32-inch to 38-inch sizes, which is why they have become the default. A poured concrete pad runs $100 to $300 or more for a small slab, depending on whether you pour it yourself or hire it out, plus the base prep underneath.
The pad is the cheap part. The real cost of replacing a pad under an existing unit is the labor to lift and reset the condenser, which usually pushes a professional pad replacement to $150 to $500. That is because the unit is heavy, still charged with refrigerant, and tethered by a lineset that cannot be yanked around. On a new install the pad is bundled into the job, so you rarely see it as a separate line item.
If your pad problem is really a symptom of a tired, tilting old system, weigh the fix against replacement. A sinking pad on a 5-year-old unit is worth correcting; a sinking pad on a 15-year-old unit that also needs repairs may be the nudge toward a new system. See how much a new AC costs for installed price ranges by type and tonnage.
Why is my AC pad sinking or unlevel, and does it matter?
A pad sinks because the ground under it settles, and it usually comes from one of a few causes: soil that was never compacted before the pad went down, erosion from a downspout or roof runoff draining beside the unit, or normal settling of backfilled dirt around a foundation. Clay soils that swell and shrink with moisture are common culprits.
A slight tilt does not matter, but a noticeable lean does. A condenser should sit close to level, because a badly tilted unit strains the refrigerant lineset, can interfere with the compressor's oil return, and on a heat pump keeps the base from draining meltwater during defrost. As a rule of thumb, if you can clearly see the unit leaning or a ball would roll off the top, it is worth correcting. A quarter-inch of slope across the pad is nothing to worry about.
Catch it early and the fix is simple. Let a unit lean for years and the lineset can work-harden and crack, the fan can rub, or the compressor can wear unevenly. Redirecting the downspout or runoff that caused the erosion is half the repair, because re-leveling a pad without fixing the drainage just buys you a year before it sinks again.
How do you level or replace an AC pad, and when should you call a pro?
Answer first: a light re-level of a composite pad is a careful owner job, but replacing a pad or moving a unit that has sunk badly is pro work. For a minor tilt, shut the power off at the outdoor disconnect, gently lift the low side just enough to slide in gravel or a composite shim, and settle the unit back down without pulling on the lines. Work slowly, keep the unit nearly upright, and never pry against the copper lineset.
Stop and call a pro when the job is bigger than a nudge. If the pad has dropped several inches, the refrigerant lines are tight or kinked, the unit needs to be lifted clear and reset, or the lineset has to be disconnected, that is licensed HVAC work, because opening the refrigerant circuit is regulated under EPA Section 608 and forcing a charged unit can kink a line and vent refrigerant. The disconnect also holds mains voltage, and the capacitor inside the unit stays charged after the power is off.
The safe division of labor: you handle the base and the drainage (compact the gravel, redirect the downspout, swap in a larger composite pad on a new install), and a pro handles anything that means lifting or moving a charged condenser. If you are also chasing a unit that will not start or is running poorly, work through why the outdoor unit is not turning on first, since a pad issue and an electrical fault are easy to confuse.
Frequently asked questions
What does an AC pad do?
An AC pad keeps the outdoor condenser level, lifts it off wet ground, spreads its weight so it does not sink into soft soil, and dampens vibration. A stable, flat pad protects the compressor and the copper refrigerant lineset, which both take strain when a unit tilts.
How much is an AC pad?
A composite AC pad costs about $30 to $90 for common 32-inch to 38-inch sizes. A small poured concrete pad runs $100 to $300 or more. Replacing a pad under an existing unit usually costs $150 to $500 installed, because most of that is the labor to safely lift and reset the charged condenser.
How thick does an AC pad need to be?
A composite AC pad is usually 2 to 3 inches thick and sits on 2 to 3 inches of compacted gravel that does the load-bearing. A poured concrete slab is typically 3 to 4 inches thick and carries the weight itself. Either way it needs to stay rigid under a unit that can weigh 100 to 200 pounds.
What should I put under my air conditioner?
Put a purpose-made pad under an air conditioner: either a poured concrete slab or a molded composite condenser pad set on a compacted gravel base. Avoid bare dirt, which turns to mud and lets the unit settle, and loose gravel alone, which spreads and washes out. Composite pads are lighter, cheaper, and will not crack in a freeze.
Do you need a concrete pad for an AC unit?
No, you do not need concrete. A lightweight composite pad on a compacted gravel base is the standard choice on most new installs, and it will not crack or heave when the ground freezes. Concrete is still a good option for very heavy units, sites with pooling water, or anywhere you want a fully permanent base.