STOIIP / GIIP Calculator: Oil and Gas Initially in Place

Calculate stock tank oil or gas initially in place from area, thickness, porosity, and water saturation, build a multi-zone model summed to a field total, or solve backward for the area, thickness, or porosity a target volume requires. Everything updates live.

The Volumetric Equation Behind STOIIP and GIIP

STOIIP, stock tank oil initially in place, and GIIP, gas initially in place, are the starting point for almost every hydrocarbon volumetric estimate, the total oil or gas resource contained in a reservoir before any production. Both follow the same basic volumetric logic, gross rock volume times porosity times hydrocarbon saturation, converted through a formation volume factor from reservoir conditions back to surface conditions. This tool covers a single quick calculation, a multi-zone field model, and a reverse solve for the parameters a target volume requires.

Start with the quick convert tab for a single oil or gas calculation from area, thickness, porosity, and water saturation. Build a multi-zone model to sum STOIIP across several reservoir zones with different properties into a field total. Or solve backward, enter a target volume and find the area, thickness, or porosity it implies.

STOIIP in barrels equals 7758 times area in acres times net pay thickness in feet times porosity times one minus water saturation, all divided by the oil formation volume factor. The constant 7758 converts acre-feet of gross rock volume into barrels, since one acre-foot equals 7758 barrels. GIIP follows the same structure using 43560, the number of cubic feet in an acre-foot, divided by the gas formation volume factor instead of the oil version.

Every term in the equation carries its own uncertainty, area and thickness usually come from seismic mapping and well correlation, porosity and water saturation come from log or core analysis, and the formation volume factor comes from a PVT study or an analog. A volumetric estimate is only as reliable as the least certain of these inputs, which is why STOIIP and GIIP are typically reported as a range rather than a single number in early field appraisal.

Building a Field Total From Multiple Zones

Real reservoirs are rarely uniform across their whole areal extent, a field is usually broken into zones or fault blocks with different net pay, porosity, or saturation, and the field total STOIIP is the sum of each zone's own volumetric calculation rather than a single blended average. Averaging area weighted properties across the whole field first and running one combined calculation can give a noticeably different answer than summing zone by zone, especially when the zones vary a lot in thickness or porosity.

This is the same reason net-to-gross and shale volume cutoffs matter earlier in the workflow, since net pay thickness feeding into the STOIIP equation should already reflect those cutoffs rather than gross interval thickness.

Solving Backward From a Target Volume

Sometimes a target STOIIP is already set, perhaps from a commercial threshold or a development plan, and the useful question becomes what area, net pay, or porosity that target would require given the other parameters. Since the STOIIP equation is a simple product, any one of area, thickness, or porosity solves directly from the other three plus a target volume, no iteration required.

Common Mistakes

Using gross interval thickness instead of net pay thickness after cutoffs, which overstates STOIIP by including the shale, tight, or wet portions of the interval that a proper net-to-gross calculation would exclude.

Applying a single field average porosity and water saturation to a multi-zone field instead of computing each zone independently and summing, which can meaningfully misstate the total when zone properties differ.

Treating STOIIP as recoverable reserves rather than oil in place, STOIIP is the total resource in the reservoir, only a recovery factor applied afterward converts it into an estimate of what can actually be produced.

Ignoring the uncertainty in area and thickness from seismic mapping, since these two terms multiply directly into the volumetric answer and are often the largest source of uncertainty in an early stage estimate, larger than porosity or saturation uncertainty in many cases.

Who Uses This

Reservoir engineers and geologists computing STOIIP or GIIP as the starting point for a reserves or resource estimate.

Petroleum economists and planners using STOIIP or GIIP ranges to screen prospects or rank development opportunities.

Students and early career geoscientists learning how the standard volumetric equation connects mapped reservoir geometry to a hydrocarbon in place estimate.

Analysts building multi-zone field models and checking how sensitive a field total is to zone by zone versus blended average assumptions.

Try It Above

Use the quick convert tab for a single oil or gas volumetric calculation, move to the multi-zone model to sum STOIIP across several reservoir zones into a field total, or use the solve target volume tab when a target STOIIP is set and you need the area, thickness, or porosity it implies.

Related Calculators and References

Continue building your workflow with the Shale Volume Calculator. For background on the formulas used above, see Wikipedia: Oil in place.

Common Questions

How is STOIIP calculated?

Multiply reservoir area, net thickness, porosity, and oil saturation, then divide by the formation volume factor to move the oil to surface conditions. The calculator on this page runs the equation in field or metric units.

What units does STOIIP use?

Commonly millions of barrels at surface conditions, converted from reservoir volume through the formation volume factor. Metric workflows report standard cubic meters instead.

What is the difference between STOIIP and reserves?

STOIIP is everything in the tank; reserves are the fraction you can actually produce economically, which is STOIIP times a recovery factor. Confusing the two flatters every discovery.