Short answer

A mid-size standby generator pad, roughly 48×36 inches and 4 inches thick, needs about 0.44 cubic yards of concrete — around 20 bags of 80 lb premix. Larger whole-home standby units need a bigger, sometimes engineer-specified pad, especially where vibration and equipment weight are a concern.

Typical generator pad sizes by unit capacity

How to size and pour a generator pad

1. Check the generator manufacturer’s installation manual for the exact minimum pad size and clearance requirements.
2. Add a couple of inches of clearance beyond the unit’s base on all sides for anchoring and service access.
3. Pick a thickness: 4 inches for most residential standby units, 4–6 inches for larger commercial units.
4. Enter your dimensions above to get the cubic yardage and bag count.
5. Excavate and compact a level base, add a gravel bed for drainage, then form and pour the pad.
6. Keep the pad slightly above surrounding grade, and maintain required clearance from windows, doors, and combustible materials per local code.

Worked example

Pad size

48 in × 36 in × 4 in thick

Volume

48/12 × 36/12 × 4/12 = 4.0 ft³, plus 10% waste = 4.4 ft³

Bags (80 lb, 0.60 ft³ each)

ceil(4.4 / 0.60) = 8 bags

Common mistakes

• Ignoring the manufacturer’s minimum pad size and clearance requirements, which can void warranties or violate code.
• Pouring the pad too thin for a heavy commercial-grade unit, leading to cracking under vibration.
• Placing the pad too close to windows, doors, or vents without checking local exhaust clearance rules.
• Skipping the gravel base, which lets the pad settle unevenly and throw off the generator’s alignment.

Assumptions

• Standard premix concrete at roughly 150 lb per cubic foot.
• Bag yields follow manufacturer averages: 80 lb = 0.60 ft³, 60 lb = 0.45 ft³, 50 lb = 0.375 ft³, 40 lb = 0.30 ft³.
• Waste percentage accounts for spillage, uneven subgrade, and minor overpour.
• Does not include gravel base material, anchor bolts, forms, or vibration-isolation pads.

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