Effective Porosity Calculator — Wilkinson Equation

How to Use This Calculator

Choose your unit system Select SI, US, Imperial UK, or CGS depending on how your well data is reported.

Enter Well Yield (Q) and Drawdown (s) These are standard values from any well completion report or pumping test. Specific Capacity is computed automatically. Alternatively, enter Q/s directly if you already have it.

Click Calculate The Wilkinson equation returns Nₑ instantly. The result includes an aquifer-type interpretation and percentage form.

Interpret the result Use the Interpretation Guide on the right to identify the likely aquifer type. Values outside 0.01–0.45 warrant a review of the input data.

Start over Use the Reset button to clear all values and results instantly, ready for the next well.

Need to calculate multiple wells at once? Use the Batch Processing section below the calculator — upload a CSV file and download results for all wells in one step.

Unit System:

Inputs

Well Yield (Q) m³/day

Pumping rate from well completion report

Drawdown (s) m

Water level decline during pumping test

Specific Capacity (Q/s) m²/day

Computed from Q and s, or enter directly

Results

Enter values and calculate to see results

Interpretation Guide

< 0.01 Very low — verify inputs

0.01 – 0.05 Fractured rock / confining beds

0.05 – 0.15 Fine-grained sediments / till

0.15 – 0.25 Sand & gravel aquifers

0.25 – 0.40 High-porosity coarse gravels

0.35 – 0.45 Karst limestone / dolomite

0.30 – 0.45 Vesicular basalt

> 0.45 Unusually high — verify inputs

Note: ranges overlap by design. Effective porosity is interpretive — the correct aquifer type assignment requires geological judgment and a well log, not mechanical table lookup.

Batch Processing

Upload a CSV file with columns Well ID, Q, and s to calculate effective porosity for multiple wells at once. The selected unit system applies to all rows.

Choose CSV File No file chosen

Expected file structure:

Well ID	Q	s
Units are taken from the unit selector at the top of the calculator. Set your unit system there before uploading.
Well-01	500	3.2
Well-02	1200	5.8
Well-03	250	12.4
Well-04	3800	1.9
Well-05	75	28.6

About This Calculator

This calculator implements the empirical power-law equation developed by Wilkinson (under review) for estimating effective porosity (Nₑ) from specific capacity (Q/s) — data routinely collected during well construction at no additional cost.

The equation achieves R² values of 0.987–0.991 across all hydrogeologic units in the Portland Basin, Oregon and Washington, and has been externally validated on approximately 5,000 wells in Texas, New Mexico, and South Sudan.

Citation

Wilkinson, J.M. (under review). "One Equation to Rule Them All: Estimating Effective Porosity from Specific Capacity Across Diverse Aquifer Types." Hydrogeology Journal.

Wilkinson, J.M. (2025). "Why Spend so Much Time and Money to Estimate Effective Porosity?" GSA Abstracts with Programs, Vol. 57, No. 6. doi:10.1130/abs/2025AM-7697

Calculate Effective Porosityfrom Specific Capacity — on the Fly