Reptile Tank Heating Calculator
Two Very Different Ways to Heat a Terrarium
Reptile enclosures need a temperature gradient, not a uniformly warm box, and the two common heat sources get there through different mechanisms: an under-tank heat mat warms substrate from below at a gentle, steady wattage, while an overhead basking bulb creates a hot spot the animal can move toward or away from. Sizing either one wrong risks an enclosure that's too cool for digestion and activity, or a basking spot hot enough to burn. This calculator sizes both methods from the enclosure's actual volume.
The Formula
Basking bulb: Wattage = Tank Gallons × (Basking Target − Ambient Temp) ÷ 20, minimum 25W
The heat-mat method uses published manufacturer gallon-to-wattage brackets rather than a continuous formula, since heat mats are sold in fixed wattage steps. The basking-bulb method instead scales with both tank volume and how many degrees above ambient room temperature the target basking spot needs to reach.
Heat Mat Sizing by Tank Volume
| Tank size (up to, gal) | Recommended heat mat |
|---|---|
| 10 | 8 W |
| 20 | 16 W |
| 30 | 24 W |
| 40 | 30 W |
| 55 | 40 W |
| 75 | 50 W |
Above 75 gallons the calculator scales at roughly 0.7 W per additional gallon beyond the 75-gallon/50W bracket.
Basking Bulb Examples
| Tank size (gal) | Basking target | Recommended bulb wattage |
|---|---|---|
| 10 | 90°F | 25 W (floor minimum) |
| 20 | 95°F | 25 W (floor minimum) |
| 40 | 100°F | 50 W |
| 75 | 105°F | 112.5 W |
Computed as tank gallons × (target − ambient) ÷ 20, with a 25W floor for small enclosures.
Where This Matters
- New enclosure setup — sizing heat sources before the animal arrives avoids a stressful mid-setup swap.
- Species with different thermal needs — a desert species needing a hot basking spot and a tropical species needing gentle, even warmth call for different methods, which this calculator handles separately.
- Seasonal ambient shifts — a room that runs colder in winter raises the required basking wattage for the same target temperature, since the formula depends on the gap between ambient and target.
How to Use This Calculator
- Enter the Tank Volume in gallons.
- Select the heating method — Under-Tank Heat Mat or Overhead Basking Bulb.
- If Basking Bulb is selected, optionally enter the Ambient Room Temperature in °F (defaults to 75°F) and enter the Basking Target Temperature in °F.
- Select Calculate to see the recommended wattage.
Related Calculations
For a different enclosure-volume calculation, see the Aquarium Volume Calculator.