The Verification Venue · a thing everyone thinks they know
The Candle That Doesn't Steal Your Air
Light a second candle across the room and the first does not burn any faster. Each candle drinks from its own pool of wax, and how fast it burns is set by the wick — not by a fight over the oxygen in the room. Below, you can operate the whole oxygen budget and watch it prove itself.
The folk intuition — "two flames, so the wax runs out in half the time" — is true, but only about the total wax you burn through, or about a single candle with two wicks feeding on one shared reservoir. It is false when you apply it to each candle's own lifetime. Two separate candles are two separate fuel tanks; the second one no more shortens the first than a second car empties your tank faster. Pick a mode and watch which number moves.
Choose what's burning
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Each candle lasts
— h
wax reservoir ÷ burn rate
Total oxygen burned
— L/hr
flames × wax rate × O₂ per gram
Room O₂ at steady state
— %
vs 20.9% ambient · flame dies ~16%
Ventilation resupplies O₂
—×
faster than the flames consume it
A modern taper burns ~0.1 g/min (Wikipedia). This is set by the wick's capillary feed, not the room.
Air changes per hour. 0.35 is the ASHRAE 62.2 minimum; real rooms leak more (0.5–1). Drag to 0 to seal the room.
Illustrative taper mass (a free choice). Only sets the lifetime numbers, not the oxygen argument.
The check — every number recomputed in front of you
These lines recompute from your sliders using the same constants the offline verifier asserts. Change anything above and watch them move.
The constants, with sources:
| constant | source | value |
|---|---|---|
| Burn rate | Wikipedia, “Candle” | 0.1 g/min |
| O₂ per g wax | CH₂ + 1.5 O₂ → CO₂ + H₂O | 2.62 L/g |
| Heat of combustion | NIST (Hamins & Bundy) | 43.8 kJ/g |
| Huggett's constant | O₂-consumption calorimetry | 13.1 kJ/g O₂ |
| Extinction O₂ | J. Chem. Educ. candle study | ~16% |
| Ventilation | ASHRAE 62.2 minimum | 0.35 ACH |
Cross-check on the O₂-per-gram figure: NIST's 43.8 kJ/g ÷ Huggett's 13.1 kJ per g-O₂ = 3.34 g O₂ per g wax = 0.104 mol/g, which agrees with the stoichiometric 0.107 mol/g to within ~2.3%. Run it yourself: node research/do-two-candles-burn-faster/verify-do-two-candles-burn-faster.mjs.
What's exactly true, what's assumed, and what I couldn't verify
Exactly true. A candle flame is a laminar diffusion flame fed by capillary action drawing molten wax up the wick; its mass burn rate is governed by that fuel supply, not by the ambient oxygen (as long as there's plenty). So two independent candles each burn at their own rate, and each lasts (its own wax) ÷ (its own rate) — a figure that does not know or care how many other candles are lit. Lighting a second candle exactly doubles the total wax and total oxygen consumed per hour, while leaving each candle's lifetime untouched. That is the whole resolution.
The one case where “burns twice as fast” is right. A single candle with two wicks feeds both flames from one shared pool of wax, so that pool empties in half the time. Retailers of three-wick jar candles say exactly this. The intuition isn't wrong; it's just about shared fuel, and gets mis-applied to two separate candles.
Assumed / free choices. The 40 m³ room is illustrative. Paraffin is approximated as (CH₂)ₙ. Molar gas volume is taken at 25 °C (24.47 L/mol; at 0 °C it's 22.4, which shifts the litres-per-gram by ~9% but changes no conclusion). Wax-per-candle (default 70 g) only sets the lifetime figures. Ventilation is the ASHRAE minimum 0.35 ACH — real rooms leak more, which only strengthens the conclusion. The sealed-room extinction time uses a fixed-volume O₂-removal approximation (combustion is roughly mole-neutral as CO₂ and water vapour replace the O₂).
Scope caveat. This holds for candles far enough apart that their flames don't merge or couple. Closely spaced flames are known to synchronise and merge, which can change the burn rate — an active area in buoyant diffusion-flame research. It is not the everyday two-candles-on-a-table case.
A related myth, named. The classic “a candle under a jar goes out because it used up all the oxygen” is itself wrong: the flame dies near ~16% O₂ — with roughly three-quarters (~77%) of the oxygen still present (only ~23% consumed) — from CO₂ build-up, heat and dilution, not oxygen exhaustion.
What I couldn't verify. A widely-repeated claim that “The Action Lab measured two candles extinguishing within 2% of a single-candle baseline” is unverifiable — I found no such experiment and do not cite it. If you know of a controlled measurement of two separate candles' individual lifetimes, that would be the direct empirical confirmation this page argues for from first principles.