Fuel density and volume-mass conversion
In every mass-and-balance calculation, fuel is expressed in mass (kg) — but the tank content is measured in volume (litres or US gallons). The fuel density bridges the two.
Standard values (ISA, 15 °C)
| Fuel | Standard density (15 °C) |
|---|---|
| AVGAS 100LL | 0.72 kg/L (0.72 g/cm³) |
| JET A-1 | 0.80 kg/L (often 0.775–0.840) |
| MOGAS (premium 95 octane) | 0.73 kg/L (typical) |
Important: these are standard values at 15 °C. Actual density varies with:
- Temperature (higher → density drops by about 0.000 7 kg/L per °C),
- Fuel batch (small differences between deliveries),
- Blend type (summer/winter, AVGAS source).
Volume units
| Unit | Conversion |
|---|---|
| 1 US gallon (USG) | 3.785 L |
| 1 Imperial gallon (Imp gal) | 4.546 L |
| 1 litre (L) | 1 dm³ |
Sample conversions:
- 53 USG × 3.785 = 200.6 L (full C172 tank).
- 30 USG × 3.785 = 113.55 L.
- 30 Imp gal × 4.546 = 136.4 L.
Volume → mass
Formula: mass (kg) = volume (L) × density (kg/L).
Examples:
- 40 USG AVGAS:
- 40 × 3.785 = 151.4 L.
- 151.4 × 0.72 = 109.0 kg.
- 100 L AVGAS:
- 100 × 0.72 = 72.0 kg.
Mass → volume
Formula: volume (L) = mass (kg) / density (kg/L).
Example: 90 kg AVGAS → 90 / 0.72 = 125 L (= 33 USG).
Temperature correction
For large deviation from 15 °C:
- Hot summer (35 °C): AVGAS density ≈ 0.71 kg/L.
- Cold winter (−10 °C): AVGAS density ≈ 0.73 kg/L.
Difference for 100 L of AVGAS: 71 kg to 73 kg → 2 kg difference.
For standard PPL flights 0.72 kg/L is a good approximation; AFMs generally use this value.
Practical application
Fuel gauge:
- Some types (C172) show contents in USG.
- Some European variants show in litres.
- Mass-and-balance requires mass → conversion needed.
Volume maximum check:
- C172: 53 USG = 200 L AVGAS ≈ 144 kg fuel.
- When MTOM is reached, the full tank cannot be used.
Typical consumption
| Aircraft | Cruise consumption |
|---|---|
| C172 | 8 USG/h ≈ 30 L/h ≈ 22 kg/h |
| PA-28-181 | 9 USG/h ≈ 34 L/h ≈ 24 kg/h |
| DA40 | 6 USG/h ≈ 23 L/h ≈ 16 kg/h |
Endurance is computed from fuel mass, consumption and TAS:
- Fuel quantity / consumption = endurance (h).
- Endurance × TAS = range (NM).
MOGAS vs AVGAS
Some types may be operated on premium auto fuel (MOGAS) instead of AVGAS 100LL — approval is via EASA STC (Supplemental Type Certificate) per model.
Advantages: much cheaper than AVGAS, lead-free. Disadvantages: higher vaporisation tendency → vapour-lock risk in summer and altitude, lower knock resistance (lower octane).
Density similar to AVGAS, but always use the AFM data or weighing record for exact density.