Oilfield Units of Measurement: Why They Matter and How to Convert

Every number in the oil and gas industry comes with a unit, and a number without its unit is meaningless. A mud weight of “10” could be 10 ppg, 10 lb/ft³ or 10 kN/m³, and the difference decides whether a well is safe or not. The industry also uses two main systems at the same time, field (US customary) units and SI (metric) units, so engineers move between them every day. This article explains why units matter, lists the units you will meet most often, gives the key conversion factors, and shows how to avoid the typical mistakes.

Why Units Matter

A wrong unit gives a wrong answer, even when the formula is perfect. Two well-known examples from outside drilling show how costly this can be:

• In 1999 NASA lost the Mars Climate Orbiter, a spacecraft worth over 100 million dollars, because one team produced results in pound-force seconds while the other expected newton seconds.
• In 1983 an Air Canada Boeing 767 ran out of fuel in flight because the fuel was calculated using pounds when the aircraft required kilograms. The crew managed to glide to a safe landing.

In drilling and production, the same kinds of error appear in a smaller way: a hydrostatic pressure calculated with meters instead of feet is wrong by a factor of three; a gas rate read in Mscf/d instead of MMscf/d is wrong by a factor of 1,000. Because well control and well design depend on pressure calculations, a unit error can lead to a kick, lost circulation, or a casing that is too weak for the pressure it will see.

Field Units and SI Units

Most of the world uses SI. The petroleum industry, however, grew mostly in the United States, where field units are still common, so both systems are in daily use. Many companies in Europe and North Africa use a mix: for example, depth in meters, pressure in bar, and mud weight in SG or kg/m³, while rigs and service companies use ppg and psi.

Common Oilfield Units and What They Measure

Key Conversion Factors

The conversion factors above are rounded for use in the field. Use more digits in a spreadsheet or program.

Handy Constants Tied to Units

Many formulas contain a constant that exists only because of the units. The constant changes when the units change:

• 0.052: psi per ft per ppg, so PH (psi) = 0.052 × ppg × ft.
• 0.433: psi/ft for fresh water (SG = 1), so PH (psi) = 0.433 × SG × ft.
• 0.0981: bar per m for SG = 1, so PH (bar) = 0.0981 × SG × m.
• ÷ 10: PH (kg/cm²) = SG × m ÷ 10.

These are all the same physics written for different units. You can test them yourself using the hydrostatic pressure calculator, which accepts the different density and depth units.

Worked Example: Converting Before You Calculate

A well has 1.2 SG mud and a TVD of 3,000 m. What is the hydrostatic pressure in psi?

1. Convert density: 1.2 SG × 8.345 = 10.01 ppg.
2. Convert depth: 3,000 m ÷ 0.3048 = 9,843 ft.
3. Calculate: PH = 0.052 × 10.01 × 9,843 ≈ 5,126 psi.
4. Convert to bar: 5,126 ÷ 14.504 ≈ 353 bar.

Check with the metric shortcut: 0.0981 × 1.2 × 3,000 ≈ 353 bar. Both agree.

What happens if you ignore the units

If you put meters straight into the field formula, 0.052 × 10 × 3,000 = 1,560 psi. The true value is about 5,126 psi, so the answer is wrong by roughly 70%. A well designed on that number would be dangerously underbalanced.

Common Unit Mistakes

• Mixing systems in one equation, such as ppg with meters.
• Gauge vs absolute pressure. psig is measured relative to the atmosphere; psia = psig + 14.7. Gas laws need absolute pressure.
• “M” and “MM”. In oilfield gas units, M means thousand and MM means million (Mscf = 1,000 scf). In SI, M means mega (million). The same letter can mean different things.
• US gallon vs imperial gallon. They are not the same. One US gallon is 3.785 L, and one imperial gallon is 4.546 L.
• Measured depth vs true vertical depth. Pressure uses TVD.
• Stock tank vs reservoir volumes. STB, scf and rb are measured at different conditions. See classification of reservoir fluids for Bo and Rs.
• Case. md is millidarcy; mD is also used, but in SI m is milli and M is mega, so always write units carefully.
• Copy-and-paste constants from an equation that was written for other units.

Best Practices

1. Always write the unit next to every number, in reports, spreadsheets and emails.
2. Convert all inputs to one consistent system before calculating.
3. Check the answer against a number you know. For example, fresh water gives 0.433 psi/ft (8.33 ppg), and a normal formation gradient is about 0.465 psi/ft; see formation pressure.
4. Do a simple order-of-magnitude check: is a pressure of 500 psi or 50,000 psi sensible at this depth?
5. Label spreadsheet columns with the units, and avoid hidden conversions inside formulas.
6. When you receive data from a vendor, confirm their units and the reference conditions (for example, standard temperature and pressure for gas).

Units in Everyday Drilling Work

Units appear in every topic on this site. For pressure terms, see key pressure definitions and types of operational pressures. For the safe mud weight, a leak-off test gives pressure in psi and the result is converted into an equivalent mud weight in ppg. Rock properties such as permeability have their own units (darcy), and fluid quality is described by API gravity, which is a unit-like scale linked to SG: API = 141.5 ÷ SG − 131.5.

Frequently Asked Questions

Why do oilfields use ppg instead of kg/m³?

Because drilling developed in the United States where mud weight is measured in pounds per gallon. Many rigs still use it, even in countries that otherwise use SI.

Is SG the same as g/cc?

For practical purposes yes: specific gravity relative to fresh water is numerically equal to the density in g/cc.

What is the difference between psi, psig and psia?

psi is a unit of pressure. psig is gauge pressure, measured relative to the atmosphere. psia is absolute pressure, which equals psig plus atmospheric pressure (about 14.7 psi).

How many barrels are in a cubic meter?

One cubic meter is about 6.29 barrels (1 bbl = 0.159 m³).

Key Takeaways

• A number without a unit is meaningless; a wrong unit can be dangerous.
• Field and SI units are both used, so you must be able to convert between them.
• Constants like 0.052 and 0.433 belong to specific units.
• Convert first, calculate second, then check the answer against a known value.