Petroleum Reserve Volume Calculator
This calculator estimates original oil in place (OOIP) using the standard petroleum engineering volumetric method. The formula, published by the Society of Petroleum Engineers (SPE), converts reservoir geometry and fluid properties to stock tank barrels at surface conditions. It is the first step in any petroleum resource assessment. OOIP multiplied by a recovery factor gives recoverable reserves.
Volumetric OOIP formula
OOIP (STB) = 7,758 x A x h x phi x (1 - Sw) / Bo
Where: 7,758 = barrels per acre-foot conversion constant; A = area (acres); h = net pay (feet); phi = porosity (fraction); Sw = water saturation (fraction); Bo = oil formation volume factor (res bbl/STB). Source: SPE Petroleum Engineering Handbook Vol. I (2006).
Petroleum reserve volume calculator: frequently asked questions
What is original oil in place (OOIP)?
Original oil in place (OOIP) is the total volume of crude oil estimated to exist in a reservoir before production begins, measured at surface conditions. It differs from recoverable reserves, which depend on recovery efficiency and economics.
What is the volumetric OOIP formula?
The volumetric method calculates OOIP = (7,758 x A x h x phi x (1 - Sw)) / Bo, where A is area in acres, h is net pay thickness in feet, phi is porosity, Sw is water saturation (fraction), and Bo is oil formation volume factor (reservoir bbl/stock tank bbl). The constant 7,758 converts acre-feet to barrels.
What is formation volume factor (FVF)?
The oil formation volume factor (Bo) accounts for the shrinkage of oil as it moves from high-pressure reservoir conditions to surface conditions. Typical Bo values range from 1.05 for heavy oil to 2.0+ for highly gas-saturated light oil. Bo is measured in reservoir barrels per stock tank barrel (res bbl/STB).
What is water saturation?
Water saturation (Sw) is the fraction of the pore space occupied by water in the reservoir. The oil-bearing pore space is (1 - Sw). Typical Sw ranges from 0.10 to 0.50. Values are determined from well log analysis or core measurements.
How does recovery factor relate to OOIP?
Recoverable reserves = OOIP x Recovery Factor. Primary recovery (natural pressure depletion) typically recovers 10-30% of OOIP. Secondary recovery (water flooding) can add 10-20%. Enhanced oil recovery (EOR) can add further increments. The SPE Petroleum Resources Management System (PRMS) defines the classification framework.
Official sources
- Society of Petroleum Engineers (SPE): SPE Petroleum Resources Management System (PRMS).
- U.S. Energy Information Administration (EIA): EIA Petroleum Data and Reserves.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.