Specific Yield Calculator — Unconfined Aquifer
Solve the specific-yield relation for any of its three unknowns — specific yield, drainable water volume or total aquifer volume — with step-by-step math, a live drainage schematic, and companion metrics including specific retention, porosity and available groundwater volume.
Vw ↑ — linear rise in Sy
At a fixed total volume, specific yield is directly proportional to the drainable water volume. Doubling Vw doubles Sy — the simplest possible relationship.
Vt ↑ — dilution effect
At fixed drainable volume, adding more aquifer material increases Vt and lowers Sy. This is why a thin productive layer in a thick sequence has a low bulk specific yield.
Grain size controls Sy
Coarse sand and gravel drain almost completely (Sy ≈ 20–30 %), while clay holds most of its water against gravity (Sy ≈ 2–5 %). Specific retention, not porosity, is what separates them.
Sy < n always
Specific yield plus specific retention equals total porosity: n = Sy + Sr. The gap Sr is water held by capillary and surface forces that will not drain under gravity.
How to Use This Calculator
The specific-yield relation links three quantities: drainable water volume Vw, total aquifer volume Vt, and specific yield Sy. Pick which one you want the tool to solve, then fill in the other two.
1. Choose what to solve for
Pick Sy (specific yield from Vw and Vt), Vw (drainable water volume from Sy and Vt), or Vt (total volume from Sy and Vw) — the tool rearranges the formula automatically.
2. Pick a quick preset, or type your own
Use a preset pill for common specific yields (5–30 %), water volumes (5–100 cm³) or total volumes (50–1,000 cm³), or drag the slider to any value you like.
3. Set the total porosity
Sand aquifers run 30–40 %, sandstone 15–25 %, gravel 25–35 %. Porosity sets the upper bound on Sy and drives the specific-retention companion metric.
4. Read the result and the schematic
The result card shows the solved value plus a classification of the aquifer. The live drainage schematic below the calculator shows a 100-cell aquifer column — drained cells drain fully, retained cells hold water, and solid cells are grains.
5. Switch to Advanced for more depth
Advanced mode adds a 1 % sensitivity check plus specific retention, porosity, drainable fraction, retained water volume, solid volume and a storativity-style estimate.
6. Convert from lab data, or copy the result
Use the laboratory conversion further down the page to get Sy directly from measured drainable and total volumes, or "Copy result" to grab the solved value.
What This Tool Offers
Built around the fundamental specific-yield relation Sy = (Vw/Vt) × 100 — the single most important parameter for estimating how much water an unconfined aquifer can actually deliver.
Solve in three directions
Go from drainable water volume and total volume to specific yield, or work backwards from a specific yield to either Vw or Vt.
Quick presets
One-click specific yields (5 / 10 / 20 / 25 / 30 %), water volumes and total volumes — plus five common porosity presets for the material property.
Live drainage schematic
A 100-cell aquifer column where each cell is 1 % of the total volume — showing drained pores, capillary-retained water, and solid grains at a glance.
Water-volume curve
Drainable water volume plotted against specific yield at your current total volume, with reference lines at 10 %, 20 % and 30 % to anchor typical aquifer behaviour.
Full companion metric set
Advanced mode reports specific retention, porosity, retained water volume, solid volume, drainable fraction and a storativity-style estimate.
Lab-data conversion
Turn measured drainable and total volumes into specific yield directly, with the retained fraction reported alongside, further down the page.
Reference Tables & Conversions
Laboratory conversion tools plus lookup tables for specific yield, specific retention and typical unconfined aquifer behaviour.
Specific yield from lab volumes
Specific retention & porosity from Sy
Specific Yield Classification
Qualitative ranges used in hydrogeology to describe the drainable fraction of an unconfined aquifer. Boundaries shift slightly between references, but the ordering is universal.
| Specific yield | Class | Typical material |
|---|---|---|
| < 2 % | Very low | Clay, silt, dense crystalline rock |
| 2 – 5 % | Low | Silty clay, fine silt |
| 5 – 10 % | Moderate–low | Fine sand, sandy silt |
| 10 – 20 % | Moderate | Medium sand, sandstone |
| 20 – 30 % | High | Coarse sand, gravelly sand |
| > 30 % | Very high | Gravel, clean coarse gravel |
Typical Specific Yield & Retention of Earth Materials
| Material | Specific yield Sy | Specific retention Sr | Total porosity n |
|---|---|---|---|
| Gravel (clean) | 25 – 35 % | 2 – 5 % | 28 – 38 % |
| Coarse sand | 20 – 30 % | 3 – 6 % | 25 – 35 % |
| Medium sand | 15 – 25 % | 5 – 10 % | 25 – 35 % |
| Fine sand | 10 – 20 % | 8 – 15 % | 25 – 35 % |
| Silt | 3 – 10 % | 15 – 30 % | 35 – 45 % |
| Clay | 1 – 5 % | 30 – 50 % | 40 – 55 % |
| Sandstone | 5 – 15 % | 5 – 15 % | 15 – 25 % |
| Limestone (karst) | 5 – 20 % | 1 – 5 % | 5 – 25 % |
| Peat | 10 – 20 % | 40 – 60 % | 60 – 80 % |
Specific Yield vs. Specific Retention at Fixed Porosity
| Sy (%) | Sr (%) if n = 30 % | Sy / n | Available water from 1 m³ (L) |
|---|---|---|---|
| 5 | 25 | 0.167 | 50 |
| 10 | 20 | 0.333 | 100 |
| 15 | 15 | 0.500 | 150 |
| 20 | 10 | 0.667 | 200 |
| 25 | 5 | 0.833 | 250 |
| 30 | 0 | 1.000 | 300 |
What Does Sy = (Vw / Vt) × 100 Measure?
Specific yield is the fraction of an unconfined aquifer's bulk volume that will drain freely under gravity. It is the single most important parameter for estimating how much water a water-table aquifer can actually deliver to a well, a spring or a drainage gallery. Unlike total porosity, which counts every void, specific yield counts only the voids that connect and drain — the rest is held by capillary and surface forces and is quantified by the specific retention Sr.
Specific Yield Formula
The defining relationship is:
Rearranged to solve for the two other variables:
Vt = Vw / ( Sy / 100 )
Specific yield is bounded above by total porosity and linked to specific retention by a simple mass-balance relation:
Specific Yield Variables
| Symbol | Meaning | Notes |
|---|---|---|
| Sy | Specific yield, expressed as a percentage | Always ≤ total porosity n |
| Vw | Drainable water volume | Water that drains under gravity from a saturated sample |
| Vt | Total (bulk) aquifer volume | Solids + all pores (drained and retained) |
| Sr | Specific retention | Water held by capillary forces — does not drain |
| n | Total porosity | n = Sy + Sr |
How Water Volume Affects Specific Yield
Specific yield responds linearly to drainable water volume at fixed total volume, and inversely to total volume at fixed drainable volume. That is why the same geological material can have very different bulk specific yields depending on how thick the saturated interval is: a thin productive sand bed sandwiched between thick clay layers has a low bulk specific yield even though the sand itself drains well. This is also why aquifer tests must be interpreted carefully — the specific yield that matters at the well is a bulk average over the whole saturated thickness.
Specific Yield Calculation Examples
Sy = ( 20 / 100 ) × 100
Sy = 20 % (Sr = n − 20 %)
Vw = ( 15 / 100 ) × 200
Vw = 30 cm³ (available water from 200 cm³ of aquifer)
Vt = 75 / ( 25 / 100 ) = 75 / 0.25
Vt = 300 cm³
Frequently Asked Questions
Specific yield is the volume of water that an unconfined aquifer will release from storage when the water table drops, expressed as a fraction of the aquifer's total bulk volume. It is the parameter that converts a measured water-table decline into a volume of water produced.
Porosity counts every void, including those sealed off from the water table and those small enough to hold water by capillary forces. Specific yield counts only the voids that drain under gravity. Specific retention is the difference: n = Sy + Sr. A clay can have 50 % porosity and only 3 % specific yield.
No. Specific yield is by definition a subset of the pore space, so Sy ≤ n. A measured value where Sy > n almost always indicates an error in one of the two measurements, or that the "porosity" value was actually an effective porosity already excluding retained water.
Medium to coarse sand aquifers typically fall between 15 % and 30 %, with 20 % a common working value. Gravel aquifers can reach 25–35 %, while silty or clay-rich units drop to 3–10 %. The reference tables on this page give the full ranges.
Three common approaches. In the laboratory column method, a saturated sample is allowed to drain under gravity and the drained water is collected and weighed. In field methods, water-table decline during a pumping test is combined with the volume pumped. In numerical methods, Sy is estimated by calibrating a groundwater model against observed water levels.
Because capillary forces hold a substantial amount of water in the smaller pores and against grain surfaces even after gravity drainage is complete. This water is the specific retention. In fine-grained material, the retained fraction can be much larger than the drainable fraction, which is why clays have high porosity but very low specific yield.
Yes, somewhat. Specific yield is a drainage-rate–dependent property: if the water table drops slowly, more water has time to drain and the apparent specific yield is larger. This is why field estimates of Sy from short pumping tests often come out lower than laboratory values obtained from long drainage columns.
Variables
| Symbol | Quantity | Unit |
|---|---|---|
| Sy | Specific yield | |
| Volume of Water Drained | Water released by gravity drainage | m3 |
| Total Volume of Aquifer | Total aquifer volume dewatered | m3 |
What it means
Specific yield (Sy) is the ratio of the volume of water that will drain from an unconfined aquifer under gravity to the total volume of the aquifer. It represents the fraction of pore space that can contribute to groundwater flow. Sy is typically less than porosity. It is used in groundwater balance calculations, for estimating water table fluctuations, and for managing groundwater resources. Understanding Sy is important for hydrogeologists to evaluate aquifer productivity and to assess sustainable yields.
Worked example
Specific Yield – Two Detailed Examples
Real‑World| Parameter | Value |
|---|---|
| Volume Drained (m³) | 15000 |
| Aquifer Volume (m³) | 100000 |
| Parameter | Value |
|---|---|
| Drained | 8000 |
| Volume | 50000 |
Common mistakes
- Specific yield Sy: The volume of water that drains from an unconfined aquifer per unit area per unit decline in water table.
- Volume drained: The water released by gravity – not all water is drainable.
- Total volume: The volume of aquifer considered – both saturated and unsaturated.
- Units: Sy is dimensionless (fraction).
- Depends on: Grain size, sorting, and degree of saturation – not a constant.
Applications
Specific yield (Sy) is the volume of water drained from an unconfined aquifer per unit decline in water table, expressed as a fraction of the total aquifer volume. This parameter is essential for estimating groundwater recharge and for modelling water‑table fluctuations. Hydrogeologists use Sy to determine the amount of water available from an unconfined aquifer for pumping, to design water‑supply wells, and to assess the impact of droughts. By measuring specific yield, professionals can better manage groundwater resources, especially in semi‑arid regions. Understanding Sy is also critical for contaminant transport and for evaluating the vulnerability of unconfined aquifers.
- Estimation of usable groundwater storage in unconfined aquifers
- Water‑table fluctuation and recharge analysis
- Design of wells and pumping schemes
- Assessment of aquifer vulnerability to contamination
- Groundwater resource management in arid and semi‑arid regions
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