If you drive a CNG vehicle in Bangladesh, you have probably noticed that the pump attendant never fills your cylinder to the stated capacity on the label. A 60-litre cylinder is never actually filled with 60 litres of gas. This is not a scam — it is a safety and physics requirement. Understanding exactly how CNG filling works, how much it costs, and how to calculate your own fill cost takes less than five minutes once you know the formula.
Why CNG Is Measured in kg, Not Litres
Petrol and diesel are liquids with a stable volume at ambient conditions, so they are measured in litres. CNG is a compressed gas — its volume shrinks dramatically under pressure (200–220 bar in a car cylinder). At these pressures, measuring volume is impractical because it changes with temperature and pressure. Instead, CNG is sold and measured by weight in kilograms, which is constant regardless of temperature or pressure. One kg of CNG contains approximately 49 MJ of energy, making it a reliable unit for pricing.
The Safe Fill Level — 80% Rule
Compressed gas cylinders have a rated water-volume capacity (e.g. 60 litres). However, regulations and cylinder manufacturers specify that cylinders should only be filled to 80% of their rated capacity. This is the safe working pressure limit and leaves headroom for gas expansion when the cylinder heats up during the day. A 60-litre cylinder is therefore filled to approximately 12–13 kg of CNG (not 60 kg — the litre rating refers to water volume, not gas weight).
CNG Weight Capacity Formula: Usable CNG (kg) = Cylinder Water Volume (L) × 0.206 × 0.80 Why 0.206? At 200 bar pressure, 1 litre of cylinder volume holds approximately 0.206 kg of CNG. Common cylinder sizes: 40-litre cylinder: 40 × 0.206 × 0.80 = 6.59 kg 60-litre cylinder: 60 × 0.206 × 0.80 = 9.89 kg 80-litre cylinder: 80 × 0.206 × 0.80 = 13.18 kg 100-litre cylinder: 100 × 0.206 × 0.80 = 16.48 kg
Fill Cost Calculation — Step by Step
Full Fill Cost = Usable CNG (kg) × CNG Price (৳/kg) Example 1: 60-litre cylinder, full fill from empty Usable CNG = 60 × 0.206 × 0.80 = 9.89 kg Fill cost = 9.89 × ৳43 = ৳425.27 Example 2: 80-litre cylinder, half-tank refill (50% → 100%) Full capacity = 80 × 0.206 × 0.80 = 13.18 kg Half fill = 13.18 × 0.50 = 6.59 kg Fill cost = 6.59 × ৳43 = ৳283.37 Example 3: 100-litre cylinder, full fill Usable CNG = 100 × 0.206 × 0.80 = 16.48 kg Fill cost = 16.48 × ৳43 = ৳708.64
How to Estimate Your Refuelling Frequency
Km per full tank = Usable CNG (kg) × Fuel Economy (km/kg) Example: 60-litre cylinder, fuel economy 18 km/kg Km per tank = 9.89 × 18 = 178 km Example: 80-litre cylinder, 20 km/kg economy Km per tank = 13.18 × 20 = 264 km Fills per month = Monthly km ÷ Km per tank 2,000 km/month ÷ 178 km/fill = 11.2 fills/month Monthly cost = 11.2 × ৳425 = ৳4,760/month
Quick Reference: Fill Cost at ৳43/kg
| Cylinder Size | Usable CNG | Full Fill Cost | Range (18 km/kg) |
|---|---|---|---|
| 40-litre | 6.6 kg | ৳284 | ~119 km |
| 60-litre | 9.9 kg | ৳425 | ~178 km |
| 80-litre | 13.2 kg | ৳567 | ~238 km |
| 100-litre | 16.5 kg | ৳709 | ~297 km |
Why Does My Meter Show Different Numbers?
CNG dispensing meters at filling stations read in kg (mass flow). If a meter shows '9.5 kg' filled, the cost is 9.5 × ৳43 = ৳408.50. The dispenser should print or display the total automatically. If a station quotes a higher total, ask for the kg reading and calculate yourself. Meter tampering is illegal under the Weights and Measures Act, but independent verification protects you.
Tip
Use the CostNest CNG & LPG Calculator Hub to enter your exact cylinder size and current fuel economy. It automatically applies the 80% safe fill rule, calculates your cost per km, and estimates your monthly fuel bill — no manual calculation needed.