Frost depth in New Hampshire
The code in New Hampshire enforces 48 inches of footing depth below finished grade. Add 6 inches of bearing and that becomes a dig depth of 54". The numbers below are the code values an inspector measures, not climate averages — the local building department always has the final word.
| Area | Frost depth | Dig depth | Set by |
|---|---|---|---|
| Statewide | 48" | 54" | NH Residential Code (IRC) Table R301.2(1) |
One number for the whole state, though the value tracks the largest cities where most permits are filed. Counties can still be stricter, so confirm with the building department before you dig.
Why New Hampshire's number looks the way it does
New Hampshire runs 48 inches under the state residential code — standard New England depth, applied from the seacoast to the White Mountains.
From frost depth to dig depth in New Hampshire
Footings bear on undisturbed soil below the frost line, so the hole goes 6 inches deeper than the code number: 48 inches of frost in New Hampshire means digging 54 inches, and a 6-foot Sonotube covers it with room to spare. A 4-inch gravel base at the bottom of the hole is common practice for drainage — add it to the tube length, not the dig depth, because the footing still has to reach 48 inches below grade.
How much concrete a footing takes in New Hampshire
At New Hampshire's 48-inch code depth, a solid 12-inch pier with no post in the hole takes about 6 bags of 80-lb mix — count on 5 for a 10-inch hole and 11 if you open it up to 16 inches. A 4x4 post displaces enough concrete to drop those counts by roughly a bag, and the fence post calculator does that subtraction per hole.
The bags calculator converts any total into 80, 60, 50 or 40-pound bags, and the concrete calculator handles slab and footing volumes.
New Hampshire frost depth FAQ
How deep is the frost line in New Hampshire?
48 inches below finished grade. NH Residential Code (IRC) Table R301.2(1) sets that number, and it applies to the areas most permits are filed in. It is the code value an inspector measures, not the average depth the ground actually froze last winter.
How deep do footings have to go in New Hampshire?
Bearing sits below the frost depth, so dig to the code depth plus 6 inches: 48 inches of frost means a 54-inch dig in New Hampshire. The extra 6 inches is the bearing layer that keeps the footing from sitting on frost-susceptible soil, and it is what NH Residential Code (IRC) Table R301.2(1) expects to see on a permit drawing.
What happens if a footing stops above the frost line in New Hampshire?
Water in the soil freezes, expands roughly nine percent, and lifts whatever rests on it — that is frost heave, and in New Hampshire the design freeze reaches 48 inches. A footing above that line moves every year: slabs crack at the control joints, decks rack, and fence posts walk out of plumb within a few seasons. The 6-inch bearing layer below the code depth is what turns a hole into a foundation.
Plan a pour in New Hampshire
Other states
Last updated: 2026-11-02