Drilling guides › Drilling unit conversions: ppg, sg, psi, bar, bbl and m³
Drilling unit conversions: ppg, sg, psi, bar, bbl and m³
Rigs often mix oilfield and metric units: a metric well plan, an American pump manual, a European mud lab. These are the factors that come up every day.
Most conversion errors on a rig are not arithmetic slips. They come from a number copied without its unit, a factor taken from the wrong row, or a "gallon" or "ton" that meant something different to the person who wrote it. Knowing where each factor comes from makes those errors easier to spot.
Mud density
| From | To | Multiply by |
|---|---|---|
| ppg | sg (g/cm³) | 0.1198 (or divide by 8.345) |
| ppg | kg/m³ | 119.83 |
| ppg | lb/ft³ | 7.4805 |
| sg | psi/ft gradient | 0.4335 |
| ppg | psi/ft gradient | 0.052 |
Specific gravity compares a fluid with water, so 1.00 sg is fresh water and 8.345 ppg is the weight of one US gallon of it. Multiply sg by 8.345 to get ppg, and by 1,000 to get kg/m³. The lb/ft³ factor is just the 7.48 gallons in a cubic foot.
Pressure
| From | To | Multiply by |
|---|---|---|
| psi | bar | 0.068948 |
| bar | psi | 14.5038 |
| psi | kPa | 6.8948 |
| MPa | psi | 145.038 |
One bar is 100 kPa, so 10 bar is 1 MPa. Gauges on a rig read gauge pressure (above atmosphere), which is what every well control sum uses. If a document writes barg or psig, it means the same thing; bara or psia adds roughly one atmosphere (1.01 bar or 14.7 psi).
Pressure gradient
| From | To | Multiply by |
|---|---|---|
| psi/ft | kPa/m | 22.62 |
| sg | kPa/m | 9.81 |
| ppg | kPa/m | 1.175 |
| sg | bar/m | 0.0981 |
Volume and flow
| From | To | Multiply by |
|---|---|---|
| bbl | m³ | 0.158987 |
| m³ | bbl | 6.2898 |
| bbl | US gal | 42 |
| bbl | ft³ | 5.6146 |
| gpm | L/min | 3.7854 |
| bbl/min | gpm | 42 |
| bbl/ft | m³/m | 0.5216 |
The oilfield barrel is 42 US gallons. A US gallon is 3.785 litres; an imperial gallon is 4.546 litres, about 20 % bigger. Older British and some Commonwealth equipment is still labelled in imperial gallons, so check before using a pump or tank chart from an unknown source. The bbl/ft to m³/m factor is 0.158987 ÷ 0.3048.
Length and diameter
| From | To | Multiply by |
|---|---|---|
| ft | m | 0.3048 |
| in | mm | 25.4 |
Weight, force and torque
| From | To | Multiply by |
|---|---|---|
| lb | kg | 0.4536 |
| klbf | kN | 4.448 |
| lb/ft (pipe weight) | kg/m | 1.488 |
| ft·lbf | N·m | 1.356 |
| US (short) ton | tonne | 0.9072 |
A "ton" on a crane chart, a cement order and a hook load display can be three different things: the short ton of 2,000 lb, the metric tonne of 1,000 kg (2,205 lb), or the long ton of 2,240 lb. Hook loads in tonnes are mass, while kN are force; for everyday use one tonne is close to 9.81 kN.
Temperature
A bottomhole temperature of 250 °F is about 121 °C. Temperature differences convert without the 32: a rise of 18 °F is a rise of 10 °C.
Worked example
A metric plan calls for 1.38 sg mud at 3,200 m TVD. In oilfield units that is 1.38 × 8.345 = 11.52 ppg at 3,200 ÷ 0.3048 = 10,499 ft, and a hydrostatic pressure of 0.052 × 11.52 × 10,499 ≈ 6,289 psi (about 434 bar).
Check it in metric directly: 0.0981 × 1.38 × 3,200 ≈ 433 bar. The two routes agree to within the rounding of the constants.
Worked example: flow rate
A mud report from a metric rig gives 2,500 L/min.
Worked example: gradient and loads
10.5 ppg mud is 10.5 × 0.1198 ≈ 1.26 sg, with a gradient of 0.546 psi/ft or about 12.3 kPa/m. A hook load of 350 klbf is about 1,557 kN, or 159 tonnes of mass, and 25,000 ft·lbf of make-up torque is about 33,900 N·m (33.9 kN·m).
Where the well control constants come from
The two constants in the hydrostatic formula are themselves conversions. In oilfield units, 0.052 is 7.48 gallons per cubic foot divided by 144 square inches per square foot, which turns ppg and feet straight into psi. In metric, 0.0981 is gravity (9.81 m/s²) divided by 100, which turns sg and metres into bar. Because both are rounded, a calculation done in oilfield units and the same one done in metric will normally differ by a fraction of a percent. That is expected and is not a sign that either answer is wrong.
Mud balance scales
A standard mud balance usually carries several scales on the same beam: ppg, sg (or g/cm³), lb/ft³ and psi per 1,000 ft. They are all the same reading in different units, so a 10.5 ppg sample also reads about 1.26 sg, 78.5 lb/ft³ and 546 psi/1,000 ft. Read the scale your sheets use and convert nothing by hand.
Depths and datums
A depth conversion is only correct if both numbers refer to the same starting point. Offshore plans are often written from mean sea level or the seabed, while drillers' depths start at the rotary table or drill floor. Converting 3,200 m below sea level to feet is easy; forgetting to add a 25 m air gap before using it in a pressure sum is the real error, and it is worth about 80 ft of mud column.
How many decimals matter
Mud weight is mixed and measured to 0.1 ppg (about 0.01 sg), and pressure gauges on the choke panel read to a few psi. Carrying six decimal places through a sum does not make it more accurate than its inputs. Keep full precision inside a calculation, then round the answer to what can actually be measured or mixed, and round kill weights up.
Common mistakes
- Mixing 8.33 and 8.345 in the same calculation, then chasing a small difference that is only rounding.
- Converting a depth in metres to feet but leaving the density in sg, so the formula's constant no longer matches.
- Reading a flow in m³/h as m³/min, or L/s as L/min.
- Rounding in the middle of a long chain of conversions instead of at the end.
Good practice
- Write the unit next to every number on a sheet, every time.
- Pick one set of units for a calculation, convert all inputs to it first, and only convert the answer back at the end.
- Cross-check important results by working them in both systems, as in the first example above.