Relative humidity is a percentage of a moving target
Relative humidity is the water in the air as a share of the most that air could hold at its temperature. Absolute humidity is the plain weight of water in a volume of air, in grams per cubic metre.
Warm air holds more. So the same grams of water read as a lower RH when the room warms up and a higher RH when it cools. Your cigars don't care about the percentage. They trade water with the air around them, and the amount in that air is what counts.
At 70°F and 70% RH the air carries 12.88 g/m³. The calculator finds the RH at your temperature that carries the same.
70/70 chart, 60 to 80°F
Each row holds the same water as 70°F at 70% RH. The third column shows what you'd get if you just kept the pack at 70% instead.
| Temp | RH to match 70/70 | g/m³ at 70% RH | Beetle risk |
|---|---|---|---|
| 60°F (15.6°C) | 97.3% | 9.3 | Very low |
| 61°F (16.1°C) | 94.1% | 9.6 | Very low |
| 62°F (16.7°C) | 91.0% | 9.9 | Very low |
| 63°F (17.2°C) | 88.0% | 10.2 | Low, slow |
| 64°F (17.8°C) | 85.1% | 10.6 | Low, slow |
| 65°F (18.3°C) | 82.4% | 10.9 | Low, slow |
| 66°F (18.9°C) | 79.7% | 11.3 | Low, slow |
| 67°F (19.4°C) | 77.2% | 11.7 | Low, slow |
| 68°F (20.0°C) | 74.7% | 12.1 | Low, slow |
| 69°F (20.6°C) | 72.3% | 12.5 | Low, slow |
| 70°F (21.1°C) | 70.0% | 12.9 | Low, slow |
| 71°F (21.7°C) | 67.8% | 13.3 | Moderate |
| 72°F (22.2°C) | 65.7% | 13.7 | Moderate |
| 73°F (22.8°C) | 63.6% | 14.2 | Moderate |
| 74°F (23.3°C) | 61.6% | 14.6 | Moderate |
| 75°F (23.9°C) | 59.7% | 15.1 | Moderate |
| 76°F (24.4°C) | 57.8% | 15.6 | High |
| 77°F (25.0°C) | 56.1% | 16.1 | High |
| 78°F (25.6°C) | 54.3% | 16.6 | High |
| 79°F (26.1°C) | 52.7% | 17.1 | High |
| 80°F (26.7°C) | 51.1% | 17.7 | High |
Below about 66°F the matching RH climbs past 80%. Don't chase it. No storage pack sells that high, and wet, cold cigars burn badly. In a cool room, a 65 or 69% pack keeps them a shade drier than 70/70, which suits most smokers fine.
Heat is the real danger: tobacco beetles
The tobacco beetle, Lasioderma serricorne, is a stored-product pest also called the cigarette beetle, and its larvae bore through cigars. University of Florida entomologists call it the most damaging pest of stored tobacco.
Temperature sets its pace. The UF/IFAS fact sheet reports:
- Development is incomplete at 62.6°F (17°C).
- About 120 days from egg to adult at 68°F (20°C).
- About 26 days at 98.6°F (37°C).
- Adults die after six days at 39.2°F (4°C).
The risk column in the chart is our banding of those figures, not a lab threshold. Under 63°F, very low. Up to 70°F, slow. 71 to 75°F, moderate. Above that, high, and it climbs every degree. So a humidor in a hot room is a beetle problem first and a humidity problem second.
How the matching RH is calculated
The calculator uses the Magnus approximation for saturation vapour pressure, with the coefficients from Alduchov and Eskridge's NOAA work:
es(T) = 6.1094 × exp(17.625 T / (T + 243.04)), with T in °C and es in hPa.
Absolute humidity is then 216.7 × (RH/100) × es / (273.15 + T), in g/m³. Fix that at the 70/70 value, solve for RH at your temperature, and you have the number above. It's physics, not a model of ours. Hygrometer error, usually a few points, is a bigger source of doubt than the formula.
Humidor temperature and humidity questions
What is the 70/70 rule for cigars?
Keep cigars at 70°F and 70% relative humidity. It is a rule of thumb, not a standard, and plenty of smokers run a little drier, around 65 to 69% RH.
What humidity should a humidor be at 75°F?
About 60% RH holds the same moisture as 70/70. Heat is the bigger worry at 75°F, though: tobacco beetles develop faster the warmer it gets, so cool the room if you can.
Is 65% humidity too low for cigars?
No. Boveda lists 65% for a drier cigar and an easier burn. At 65°F, 65% RH is actually drier than it looks, because cool air holds less water.
Why does my humidor read higher when the room cools down?
The water in the box stayed the same while the air got colder, and cold air holds less. Same moisture, higher relative reading. It drops again as the room warms.
Where the 70/70 maths and beetle data come from
- Improved Magnus form approximation of saturation vapor pressure (Alduchov and Eskridge), NOAA National Climatic Data Center, via OSTI
- Clausius–Clapeyron relation: August–Roche–Magnus formula, Wikipedia
- Humidity: absolute and relative, Wikipedia
- What Boveda RH do I need for my cigars?, Boveda
- Cigarette Beetle, Lasioderma serricorne (EENY-227), University of Florida IFAS Extension