Aquifer Transmissivity Calculator

• Conductivity–Thickness: T = K × b, where K is hydraulic conductivity and b is saturated thickness.
• Thiem (steady-state, confined): T = (Q ln(r2/r1)) / (2π (s2 − s1)), using drawdowns at two observation radii.
• Cooper–Jacob (straight-line): T = (2.3 Q) / (4π Δs), where Δs is drawdown change per log cycle of time.

1. Select the method that matches your data source.
2. Enter values and choose units for each input.
3. Press Calculate to display results above the form.
4. Use Download CSV or Download PDF to export.
5. Review the details list to confirm conversions and equations.

• Higher T usually indicates greater well yield for the same drawdown.
• Lower T suggests tighter materials, requiring more drawdown for pumping.
• Compare results from multiple methods when possible.
• Ensure drawdown data reflect the intended flow regime and time window.

Transmissivity (T) describes how much water an aquifer can transmit through its full saturated thickness. It combines material permeability and thickness into one field-ready metric. In practice, higher T generally supports higher well yields with less drawdown, while lower T indicates tighter formations and larger pumping impacts for the same discharge.

Hydraulic conductivity spans many orders of magnitude. Clean gravel and coarse sand often fall near 10⁻³ to 10⁻² m/s, while fine sands may be closer to 10⁻⁵ to 10⁻⁴ m/s. Silts commonly drop to about 10⁻⁸ to 10⁻⁶ m/s, and clays may reach 10⁻¹¹ to 10⁻⁹ m/s. Multiplying by thickness quickly turns these into transmissivity estimates for screening.

For a fixed conductivity, transmissivity increases linearly with saturated thickness. For example, if K = 5 m/day and b rises from 10 m to 30 m, T increases from 50 to 150 m²/day. This is why partially penetrating wells, seasonal water-table changes, or layered aquifers can produce noticeably different yields even when the sediment type stays the same.

The Thiem equation estimates T from steady drawdown differences measured at two radii around a pumping well. Because the relationship contains ln(r2/r1), spacing observation wells farther apart can increase stability, as long as both remain within the same aquifer and flow regime. Accurate, steady pumping rate data are essential, and drawdowns should be taken after trends flatten.

The Cooper–Jacob approximation uses the slope Δs from drawdown plotted against log(time). The straight-line segment typically appears after early-time wellbore storage effects fade. Sampling at consistent intervals, recording time precisely, and ensuring constant discharge improve the slope estimate. A small error in Δs can materially change T because T is inversely proportional to that slope.

Transmissivity is often reported in m²/day for hydrogeology reports and in ft²/day for many engineering standards. This calculator converts internally to m²/s, then returns m²/day and ft²/day for quick comparison. When comparing studies, confirm that reported “K” values are not mixed with “T” values and that thickness refers to the same saturated interval.

While site-specific factors dominate, T values below roughly 10 m²/day often indicate limited productivity, whereas values above about 100 m²/day commonly support higher-capacity supply wells. Highly transmissive alluvial or fractured systems may exceed 1000 m²/day. Use these as screening indicators, not pass–fail rules, and pair them with water-quality, sustainability, and boundary assessments.

Confirm that r1 and r2 are measured from the pumping well center, and that drawdowns use the same datum. If Thiem returns a negative or unrealistic T, recheck whether s2 − s1 has the correct sign and whether the aquifer is truly confined. For Cooper–Jacob, verify that Δs comes from the straight-line segment and not from early transient curvature or late boundary effects.