Converter · °C = (°F − 32) × 5/9
°F to °C converter
HVAC design temperatures plus a delta-T calculator for Manual D, load, and derating math. Type in either field; presets cover the temps you actually use.
Last updated: October 2026
Converter · °C = (°F − 32) × 5/9
°F ↔ °C + ΔT
HVAC design temperatures plus delta-T for Manual D, load, and drop math.
HVAC presets (°F)
ΔT: —
°F to °C — exact formulas and delta-T
°C = (°F − 32) × 5/9. Reverse: °F = °C × 9/5 + 32. Kelvin: K = °C + 273.15. The 32 offset aligns the freezing points (32°F = 0°C); the 5/9 factor scales the degree sizes (180°F span = 100°C span). −40°F = −40°C is the only point where the scales agree. Mental shortcut: (°F − 30) ÷ 2 ≈ °C within ~2° for weather range.
Delta-T: drop the 32
For differences (not absolute temps), the offset cancels: Δ°C = Δ°F × 5/9. A 20°F supply-return delta = 11.1°C delta. Enter any two temps above for the ΔT in both scales — the number that drives Manual D friction rate, coil selection, and voltage-drop ambient correction (attic 122°F wire uses ×0.75–0.82 factors).
FAQ
Temperature conversion questions, answered
What is the formula for °F to °C?
°C = (°F − 32) × 5/9. Reverse: °F = °C × 9/5 + 32. For differences (delta-T), drop the 32: Δ°C = Δ°F × 5/9. A 20°F delta = 11.1°C delta.
What is delta-T and why does it matter?
Delta-T is a temperature difference, not an absolute reading. It drives Manual D friction-rate math, coil selection, and load calculations. Because the 32 offset cancels, Δ°C = Δ°F × 5/9 — a 20°F supply-return split is an 11.1°C split.
Is there a quick mental °F to °C trick?
(°F − 30) ÷ 2 ≈ °C, within ~2° across weather range. 75°F → ~22°C (exact 23.9). Good for the field; use the exact formula for design work.
What attic temperature should I use for derating?
122°F (50°C) is the common summer-peak assumption for residential attics. At 122°F, Table 310.15(B)(1) correction runs ×0.71–0.82 depending on insulation column — check the wire-size calculator with your exact conditions.
Does this converter work offline?
Yes. FieldCalc is an installable PWA — after your first visit, this converter runs with zero signal. All math stays on your device.
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Next step: use design temps in duct sizing ·use ambient in voltage drop ·check attic derating