Ensure a safe installation. Calculate minimum required distances from combustible walls and learn how to use heat shields to reduce clearances effectively.
Enter your stove's rated output in BTU per hour, pick the heat-shield type protecting the nearby surface, and set the distance at which you want the radiant intensity evaluated. The starting point is the NFPA 211 baseline for an unlisted single-wall stove: 36 inches (914 mm) from the stove body to a combustible wall and 18 inches (457 mm) from a single-wall flue connector to that wall.
Shield reductions follow NFPA 211 Table 12.6.2.3. A 24-gauge sheet-metal shield mounted with a 1-inch ventilated air gap earns the full 66% reduction (factor 0.34), shrinking the wall figure from 36 to about 12.2 inches and the connector figure from 18 to about 6.1 inches. A metal panel without the air gap, or cement board, is limited to 50% (18 and 9 inches). Ceiling clearance starts at 36 inches and is never reduced below 18 inches no matter which shield is selected.
The radiant-intensity readout converts stove output to watts (1 BTU/hr = 0.29307 W) and spreads that power over a hemisphere, dividing by 2πr², because a stove radiates mostly forward into the room. A 40,000 BTU/hr stove, about 11,723 W, lands roughly 829 W/m² on a surface 1.5 metres away. These results are informational baselines: a listed stove's label and your local building code always take precedence.
The largest reduction in NFPA 211 Table 12.6.2.3 modeled here is 66%, and it specifically requires a sheet-metal shield spaced off the wall with a 1-inch ventilated air gap, which turns the 36-inch wall clearance into roughly 12.2 inches. A bare metal panel or cement board without that gap qualifies for at most 50%, or 18 inches. The air gap is the active ingredient: circulating air carries heat away instead of letting it conduct straight into the combustible wall.
Rising convection and hot flue gases concentrate heat directly above a stove, so the overhead direction gets a hard floor. The calculator applies your shield factor to the 36-inch ceiling baseline but clamps the result at a minimum of 18 inches; even the best air-gap shield, which would arithmetically give 12.2 inches, still reports 18.
It estimates how many watts of radiant heat cross each square metre at the distance you entered, using a hemispherical spread (2πr²) because wood stoves emit predominantly forward. Intensity falls with the square of distance, so doubling the gap quarters the load on the facing surface, which is exactly the physics behind keeping combustibles at the listed clearance distances.