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Drilling fluids · unit conversion · field and metric units
Mud weight conversion: ppg, SG, kg/m³, lb/ft³ and pressure gradient (psi/ft)
To convert mud weight, multiply ppg by 0.052 to get the pressure gradient in psi/ft, divide ppg by 8.345 to get specific gravity, and multiply ppg by 119.83 to get kg/m³. A 12.0 ppg mud is 0.624 psi/ft, 1.438 SG and 1,438 kg/m³. Going the other way, psi/ft ÷ 0.052 gives ppg, and SG × 9.81 gives the gradient in kPa/m. This page explains where each factor comes from, gives a conversion table from 8.33 to 18.0 ppg, and works through converting pore pressure, a metric mud programme and a leak-off result.
Reference material only. Conversions are exact arithmetic; rounding conventions on your rig (for example 8.33 instead of 8.345 for water) can shift the last digit. For kill mud weights, always round up.
1. The conversion factors
ppg to pressure gradient
psi/ft = 0.052 × ppg
ppg = psi/ft ÷ 0.052 = 19.23 × psi/ft
ppg to specific gravity
SG = ppg ÷ 8.345
ppg = SG × 8.345
ppg to kg/m³
kg/m³ = ppg × 119.83
kg/m³ = SG × 1,000
ppg to lb/ft³
lb/ft³ = ppg × 7.4805
Metric pressure gradient
kPa/m = SG × 9.81
kPa/m = psi/ft × 22.62
Equivalent mud weight
EMW (ppg) = P (psi) ÷ (0.052 × TVD ft)
SG = P (kPa) ÷ (9.81 × TVD m)
Where the numbers come from: a US gallon is 231 cubic inches. A column of fluid 1 ft (12 in) tall with a 1 in² base holds 12 in³, which is 12 ÷ 231 = 0.051948 gallons, so each ppg of mud weight adds 0.051948 lb on every square inch for every foot of height; that is the 0.052 constant. A US gallon is 3.7854 litres and a pound is 0.45359 kg, so 1 ppg = 0.45359 ÷ 3.7854 = 0.11983 kg/L = 119.83 kg/m³. Fresh water at its densest (1,000 kg/m³, SG 1.000) is therefore 1,000 ÷ 119.83 = 8.345 ppg. A cubic foot holds 7.4805 gallons, hence the lb/ft³ factor. In metric units the gradient of a fluid with SG 1.000 is 1,000 kg/m³ × 9.80665 m/s² = 9.81 kPa/m, which is why SG × 9.81 gives kPa/m (and SG × 0.0981 gives bar/m).
Rig sheets often use 8.33 ppg for water instead of 8.345. 8.33 ppg is closer to fresh water at room temperature; 8.345 ppg is the exact equivalent of SG 1.000. Dividing by 8.33 gives an SG about 0.2% higher, around 0.003 SG on a 12 ppg mud. Either is acceptable as long as the whole calculation uses the same one.
2. Mud weight conversion table
Computed with 1 ppg = 119.83 kg/m³, 0.051948 psi/ft per ppg and g = 9.80665 m/s².
| ppg | SG | kg/m³ | lb/ft³ | psi/ft | kPa/m |
|---|---|---|---|---|---|
| 8.33 | 0.998 | 998 | 62.3 | 0.433 | 9.79 |
| 8.60 | 1.031 | 1,031 | 64.3 | 0.447 | 10.11 |
| 9.00 | 1.078 | 1,078 | 67.3 | 0.468 | 10.58 |
| 9.50 | 1.138 | 1,138 | 71.1 | 0.494 | 11.16 |
| 10.00 | 1.198 | 1,198 | 74.8 | 0.519 | 11.75 |
| 10.50 | 1.258 | 1,258 | 78.5 | 0.545 | 12.34 |
| 11.00 | 1.318 | 1,318 | 82.3 | 0.571 | 12.93 |
| 11.50 | 1.378 | 1,378 | 86.0 | 0.597 | 13.51 |
| 12.00 | 1.438 | 1,438 | 89.8 | 0.623 | 14.10 |
| 12.50 | 1.498 | 1,498 | 93.5 | 0.649 | 14.69 |
| 13.00 | 1.558 | 1,558 | 97.2 | 0.675 | 15.28 |
| 14.00 | 1.678 | 1,678 | 104.7 | 0.727 | 16.45 |
| 15.00 | 1.797 | 1,797 | 112.2 | 0.779 | 17.63 |
| 16.00 | 1.917 | 1,917 | 119.7 | 0.831 | 18.80 |
| 17.00 | 2.037 | 2,037 | 127.2 | 0.883 | 19.98 |
| 18.00 | 2.157 | 2,157 | 134.6 | 0.935 | 21.15 |
8.33 ppg is fresh water and 8.6 ppg is close to seawater. Pure barite at the API minimum specific gravity of 4.20 is about 35 ppg, the figure used in the barite weight-up formula.
3. Reference gradients worth remembering
| Fluid or condition | ppg | SG | psi/ft |
|---|---|---|---|
| Fresh water | 8.33 | 1.00 | 0.433 |
| Seawater (SG about 1.025) | 8.55 | 1.025 | 0.444 |
| Normal pore pressure, North Sea (commonly quoted) | 8.7 | 1.04 | 0.452 |
| Normal pore pressure, US Gulf Coast (commonly quoted) | 8.94 | 1.07 | 0.465 |
| Overburden rule of thumb | 19.2 | 2.30 | 1.0 |
Normal pore pressure is the hydrostatic pressure of the formation water, so it depends on its salinity; the values above are the figures commonly quoted in drilling textbooks for those basins. The 1.0 psi/ft overburden figure is a rough average for sediments and is replaced by density-log data in real pore-pressure work.
4. Worked example: pore pressure to equivalent mud weight
A formation at 12,000 ft TVD has a pore pressure of 6,500 psi.
- Gradient = 6,500 ÷ 12,000 = 0.542 psi/ft
- Equivalent mud weight = 0.542 ÷ 0.052 = 10.42 ppg, the same as 6,500 ÷ (0.052 × 12,000)
- Specific gravity = 10.42 ÷ 8.345 = 1.249 SG, or 1,249 kg/m³
- A mud weight of 10.42 ppg only balances the formation. With an illustrative 200 psi overbalance the mud weight becomes (6,500 + 200) ÷ (0.052 × 12,000) = 10.74 ppg, rounded up to 10.8 ppg.
5. Worked example: a metric mud programme in field units
A programme calls for 1.30 SG mud to 2,800 m TVD, and the rig works in ppg and feet.
- Mud weight = 1.30 × 8.345 = 10.85 ppg (10.83 ppg if your sheet uses 8.33)
- Depth = 2,800 ÷ 0.3048 = 9,186 ft
- Gradient = 0.052 × 10.85 = 0.564 psi/ft, or 1.30 × 9.81 = 12.75 kPa/m
- Hydrostatic pressure = 0.052 × 10.85 × 9,186 = 5,183 psi; in metric, 12.75 × 2,800 = 35,700 kPa = 357 bar, which agrees with the field-unit answer to within rounding (1 psi = 6.895 kPa)
The hydrostatic pressure formula page covers deviated wells and columns of mixed fluids.
6. Worked example: leak-off pressure to equivalent mud weight
A leak-off test at a casing shoe at 6,000 ft TVD with 10.0 ppg mud leaks off at a surface pressure of 1,100 psi.
- LOT equivalent mud weight = 10.0 + 1,100 ÷ (0.052 × 6,000) = 10.0 + 3.53 = 13.53 ppg
- Fracture gradient at the shoe = 0.052 × 13.53 = 0.703 psi/ft, or 13.53 ÷ 8.345 × 9.81 = 15.9 kPa/m
How that result turns into a MAASP for the next hole section is explained on the MAASP and leak-off test page, and the circulating version of the same idea is ECD.
7. Other units you may meet
- g/cm³ and kg/L are numerically equal to SG: 1.20 g/cm³ = 1.20 kg/L = 1.20 SG.
- lb/ft³ (pcf) is used for cement slurries and some older mud reports: 15.8 ppg cement = 118.2 lb/ft³.
- bar/10 m is sometimes used in Europe: SG × 0.981 = bar per 10 m, so a 1.50 SG mud is 1.47 bar/10 m.
- Mud gradient in psi/1,000 ft is the psi/ft figure × 1,000: a 10.0 ppg mud is 520 psi per 1,000 ft.
- API gravity is used for oils, not muds: SG = 141.5 ÷ (131.5 + °API).
8. Mistakes to avoid
- Mixing 8.33 and 8.345 for water within the same calculation.
- Dividing a pressure by measured depth instead of TVD when working out an equivalent mud weight.
- Converting metres to feet with 3.3 instead of 3.2808 on a deep well: at 4,000 m the error is about 77 ft.
- Reading an imperial gallon (4.546 L) mud weight as US ppg. Oilfield ppg always means US gallons.
- Rounding a kill or balance mud weight down after converting from SG. Round up to the next 0.1 ppg.
How DrillX helps
DrillX's engineering calculators work in field units and update as you type: hydrostatic pressure, ECD, annular pressure loss, MAASP from leak-off test, triplex pump output, annular capacity and velocity, buoyancy and string weight, and a Driller's and Wait & Weight kill sheet, in English or Arabic. DrillX has no separate unit-conversion calculator; the factors on this page are the ones to use by hand. DrillX is in final testing; email info@drillx.app for a demo or early access, or see what DrillX does.
Questions
How do you convert ppg to specific gravity?
Divide the mud weight in pounds per gallon by 8.345, the weight in ppg of fresh water with a specific gravity of exactly 1.000. A 12.0 ppg mud is 12.0 ÷ 8.345 = 1.438 SG. Many rig sheets use 8.33 instead of 8.345, which gives an SG about 0.2% higher; keep to one convention in a calculation.
How do you convert ppg to psi/ft?
Multiply the mud weight in ppg by 0.052. A 10.0 ppg mud has a pressure gradient of 0.52 psi/ft. The 0.052 constant is 12 ÷ 231: a column of fluid one foot high and one square inch in area holds 12 cubic inches, which is 12 ÷ 231 of a US gallon.
How do you convert a pressure gradient in psi/ft to mud weight?
Divide the gradient by 0.052, or multiply it by 19.23. A gradient of 0.465 psi/ft is 0.465 ÷ 0.052 = 8.94 ppg, which is usually rounded to 9.0 ppg. In metric units, kPa/m divided by 9.81 gives the specific gravity.
How do you convert ppg to kg/m³?
Multiply by 119.83. One pound per US gallon is 119.83 kg/m³, so a 10.0 ppg mud is about 1,198 kg/m³ and a specific gravity of 1.198. To go back, divide kg/m³ by 119.83.
What is equivalent mud weight?
Equivalent mud weight (EMW) is a pressure at a depth expressed as the mud weight that would create it: EMW in ppg = pressure in psi ÷ (0.052 × TVD in ft). Pore pressure, fracture pressure, leak-off test results and ECD are all commonly quoted this way, so they can be compared directly with the mud weight in the hole.
What is the pressure gradient of fresh water and seawater?
Fresh water at about 8.33 ppg has a gradient of about 0.433 psi/ft (9.79 kPa/m). Seawater, at a specific gravity of about 1.025, is about 8.55 ppg or 0.444 psi/ft. A commonly quoted normal pore-pressure gradient for the US Gulf Coast is 0.465 psi/ft, equal to about 9.0 ppg.
Sources and further reading
- NIST Special Publication 811, Guide for the Use of the International System of Units (SI), appendix B: 1 US gallon = 3.785412 L, 1 lb = 0.4535924 kg, 1 psi = 6.894757 kPa, 1 ft = 0.3048 m, standard gravity 9.80665 m/s².
- SLB Energy Glossary, mud weight: mud density in ppg, kg/m³ or as a pressure gradient in psi/ft.
- SLB Energy Glossary, pore pressure: normal hydrostatic pore-pressure gradients and their dependence on formation-water salinity.
- A. T. Bourgoyne Jr. et al., Applied Drilling Engineering, SPE Textbook Series Vol. 2 (1986), chapters 4 and 6: hydrostatic pressure in field units and normal formation-pressure gradients.
- N. J. Lapeyrouse, Formulas and Calculations for Drilling, Production and Workover, 2nd ed. (Gulf Professional Publishing, 2002): field conversion factors for mud weight and pressure gradient.
Reference summary only. Worked examples are illustrative arithmetic; the overbalance figure is an example, not a recommendation. Formation pressure and mud weight are set by the well programme.