About the Oxygen Tank Duration Calculator
This calculator uses the cylinder-factor method, the standard formula taught in respiratory therapy and EMS training to estimate how long a compressed oxygen cylinder will run at a given flow rate. It converts remaining gauge pressure into usable gas volume, then divides by the flow rate to get run time.
The formula
Duration in minutes equals the usable pressure (current tank pressure minus a safety reserve pressure) multiplied by the cylinder factor, divided by the flow rate:
Duration (min) = (Tank pressure − Reserve pressure) × Cylinder factor ÷ Flow rate (L/min)
The cylinder factor is a conversion constant, in liters of gas per psi of pressure, specific to the physical size of the tank. It comes from the tank's water volume and is published in reference tables for standard cylinder sizes:
- D cylinder: 0.16 L/psi
- E cylinder: 0.28 L/psi (the most common portable size)
- M cylinder: 1.56 L/psi
- G cylinder: 2.41 L/psi
- H or K cylinder: 3.14 L/psi (large stationary tanks)
Why a safety reserve?
A safety reserve pressure — commonly 200 psi — is subtracted before calculating usable duration. This margin exists because a cylinder should never be run down to zero: pressure gauges become less reliable near empty, flow can become inconsistent as pressure drops, and there is a risk of contaminants being drawn back into the regulator. The reserve pressure represents "empty" for planning purposes, even though a small amount of gas remains in the tank below that line.
Worked example
An E cylinder (factor 0.28 L/psi) reads 2000 psi and is running at 2 L/min, with a 200 psi reserve. Usable pressure is 2000 − 200 = 1800 psi. Usable volume is 1800 × 0.28 = 504 liters. Duration is 504 ÷ 2 = 252 minutes, or 4 hours 12 minutes, before the tank reaches the reserve line.
Limits of the estimate
This is a planning estimate, not a clinical guarantee. Published cylinder factors are nominal and can vary slightly between manufacturers, gauge accuracy degrades at low pressure, and actual flow delivery depends on the regulator and device in use. Always follow your facility's or device's own tank-change policy, and carry a margin beyond the calculated time for real-world use, especially during transport.
Frequently Asked Questions
What formula does this calculator use?
What is a cylinder factor?
Why subtract a reserve pressure before calculating?
How accurate is this estimate?
Common Tip Percentages
| Service | Typical Tip Range | Notes |
|---|---|---|
| Restaurant (sit-down) | 15–20% | 20%+ for exceptional service |
| Food delivery | 10–20% | Consider extra for long distances |
| Taxi / rideshare | 10–20% | More for assistance with luggage |
| Hotel housekeeping | $2–5/night | Leave daily, not just at checkout |
| Barber / hair salon | 15–20% | On pre-discount total |
| Pizza delivery | 10–15% or $3–5 minimum | Higher in bad weather |
General Household Budget Ratios (50/30/20 Rule)
| Category | % of Net Income | Examples |
|---|---|---|
| Needs | 50% | Rent, groceries, utilities, insurance |
| Wants | 30% | Dining out, entertainment, hobbies |
| Savings / Debt | 20% | Emergency fund, retirement, extra payments |
Common Measurement Conversions for Everyday Use
| Quantity | US / Imperial | Metric Equivalent |
|---|---|---|
| 1 cup | 8 fl oz | 237 mL |
| 1 pint | 16 fl oz | 473 mL |
| 1 quart | 32 fl oz | 946 mL |
| 1 gallon | 128 fl oz | 3.785 L |
| 1 lb | 16 oz | 453.6 g |
| 1 ft | 12 in | 30.48 cm |