Net-to-Gross Ratio Calculator

Calculate net-to-gross ratio from net and gross thickness, build a layered model that applies shale, porosity, and water saturation cutoffs to flag net reservoir intervals, or solve backward for the net or gross thickness a target ratio requires. Everything updates live.

Net-to-Gross From Thickness

Net Reservoir Thickness (m)

Gross Interval Thickness (m)

Net-to-Gross Ratio

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Non-Net Thickness

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Net-to-gross ratio is simply net reservoir thickness divided by gross interval thickness. Net reservoir usually means the portion of the interval that passes shale volume, porosity, and water saturation cutoffs, the layered model tab below applies those cutoffs directly to a set of layers rather than requiring a pre-summed net thickness.

Layered Net Reservoir Model

Max Shale Volume Cutoff

Min Porosity Cutoff

Max Water Saturation Cutoff

Top	Base	Vshale	Porosity	Sw	Flag

Each layer is flagged net only if it passes all three cutoffs at once, shale volume at or below the max cutoff, porosity at or above the min cutoff, and water saturation at or below the max cutoff. Net-to-gross ratio is the sum of net layer thickness divided by the sum of gross layer thickness across the whole interval.

Net Reservoir vs Non-Net by Layer

Net reservoir Non-net

Overall Net-to-Gross

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Gross Thickness

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Net Thickness

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Solve for Net or Gross Thickness

Net Reservoir Thickness (m)

Gross Interval Thickness (m)

Target Net-to-Gross Ratio

Solved Net Thickness

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Choose whether net or gross thickness is unknown, the other value stays editable along with the target ratio, and the missing thickness solves directly, no iteration needed. Useful for checking how much net reservoir a target ratio requires across a given gross interval, or the gross interval a known net thickness implies at a target ratio.

What Net-to-Gross Actually Filters Out

A gross interval typically includes some mix of clean reservoir rock, shale streaks, tight non-reservoir zones, and sometimes wet rock that will not contribute economically even though it is porous. Net-to-gross ratio strips all of that down to the portion that meets a defined standard, expressed as a single fraction between 0 and 1. A ratio of 0.75 means three quarters of the gross interval counts as net reservoir, the remaining quarter does not meet the cutoffs applied.

The three most common cutoffs are a maximum shale volume, since shale reduces both porosity and permeability, a minimum porosity, since very tight rock will not flow at a meaningful rate regardless of shale content, and a maximum water saturation, since a layer that is porous and clean but essentially all water bearing does not add producible hydrocarbon volume. A layer only counts as net if it passes every cutoff applied, failing any one of them excludes it.

Diagram showing shale volume, porosity, and water saturation cutoffs filtering a gross interval down to net reservoir

Choosing Cutoffs Matters as Much as the Math

The net-to-gross calculation itself is trivial once cutoffs are chosen, the real judgment call is what those cutoffs should be. Cutoffs are usually calibrated against core data or production performance in a given field or formation, a shale volume cutoff of 40 percent is a common starting point but fields with different shale mineralogy or diagenesis can need very different values. Applying a generic textbook cutoff without local calibration is one of the more common ways net-to-gross estimates end up misleading.

Tightening any one cutoff, dropping the maximum shale volume, raising the minimum porosity, or lowering the maximum water saturation, will always reduce or hold constant the resulting net-to-gross ratio, never increase it. Running the same layer set through a couple of different cutoff combinations is a quick way to see how sensitive a particular interval is to the choice of cutoff.

Column chart of five reservoir layers showing which pass shale, porosity, and water saturation cutoffs to count as net reservoir

Solving Backward for a Target Ratio

Sometimes the direction runs in reverse, a target net-to-gross ratio is assumed from an analog field or a development plan, and the question is how much net or gross thickness that ratio implies. Since the ratio is a simple division, either net or gross thickness solves directly from the other value and the target ratio, no iteration required.

Diagram showing net-to-gross ratio solved for any one of net-to-gross, net thickness, or gross thickness given the other two

Common Mistakes

Applying a single set of cutoffs across an entire field without adjusting for lithology or fluid changes between wells or zones, when different zones may need different cutoffs entirely.

Summing net thickness from a single log curve, typically gamma ray alone, without also checking porosity and water saturation, since a low shale reading does not guarantee adequate porosity or acceptable water saturation.

Reporting net-to-gross without stating the cutoffs used, since the same interval can produce meaningfully different ratios under different assumptions, making the cutoffs part of the result, not just supporting detail.

Treating net-to-gross as a fixed rock property rather than a cutoff dependent construct, when the same physical interval can have several valid net-to-gross values depending on what standard is being applied.

Who Uses This

Petrophysicists and geologists computing net reservoir thickness from log derived shale volume, porosity, and saturation curves.

Reservoir engineers using net-to-gross as an input to volumetric hydrocarbon in place calculations.

Development planners comparing net reservoir thickness across wells or fault blocks to rank drilling locations.

Students and early career geoscientists learning how cutoff based net reservoir definitions connect log data to volumetric estimates.

Try It Above

Use the quick convert tab when net and gross thickness are already known, move to the layered model to apply shale, porosity, and water saturation cutoffs across a set of layers and see the resulting ratio build up layer by layer, or use the solve thickness tab when a target ratio is assumed and you need the net or gross thickness it implies.

Related Calculators and References

Continue building your workflow with the Water Saturation Calculator. For background on the formulas used above, see Schlumberger Energy Glossary: pay.