WaterBoD-I-Y App

Water Retention Calculators
Water Retention Calculators

Water Retention Calculators

💧 Rain Catchment Calculator
Calculate rainwater collection from rectangular, triangular, and circular surfaces
🏗️ Cistern Sizing Calculator
Determine cistern capacity needed for storm events and drought periods
🛢️ Tank Capacity Calculator
Calculate storage capacity of cylindrical and rectangular tanks
📊 Water Pressure Calculator
Calculate gravity-fed water pressure and weight of stored water
🏔️ Embankment Calculator
Calculate embankment dimensions for water retention structures
📐 Slope Calculator
Calculate slope percentage, angle, and ratio for land assessment
🌱 Soil Type Calculator
Determine your soil type from sand, silt, and clay percentages
🏞️ Acre-Feet Calculator
Calculate water volume for ponds, reservoirs, and irrigation planning

Rain Catchment Calculator

Catchment Shape
Length
Width
Rainfall
Runoff Coefficient
Enhanced Runoff Coefficient Reference:
Roofs & Impervious Surfaces:
• Metal/Pitched Roofs: 0.95 (5% loss)
• Concrete/Asphalt Roofs: 0.90 (10% loss)
• Built-up Tar & Gravel: 0.80-0.85 (15-20% loss)
• Concrete/Asphalt Pavement: 0.70-0.90
• Gravel Surface: 0.40-0.60
Pervious Surfaces (Southwestern US):
• Sonoran Desert (healthy): 0.30-0.50
• Bare Earth: 0.35-0.55
• Grass/Lawn: 0.10-0.25
• Forest/Woods: 0.10-0.30

Cistern Sizing Calculator

Calculation Type
Formula: Catchment Area × Storm Rainfall × 7.48 gal/ft³ × Runoff Coefficient
Catchment Area
Storm Rainfall
Runoff Coefficient
Storm Event Sizing:
This calculates the minimum cistern size needed to capture runoff from a large storm event without overflow loss.
Consider local storm patterns - some areas may receive 3+ inches in a single event.

Tank Capacity Calculator

Tank Shape
Formula: π × radius² × effective height × 7.48 gal/ft³
Input Method
Radius
Total Height
Effective Height (usable water height)
Effective Height Notes:
• Subtract outlet pipe height from bottom (typically 4-6 inches)
• Subtract overflow pipe depth from top (typically 4-6 inches)
• Example: 8 ft tank - 4 in outlet - 5 in overflow = 7.25 ft effective

Water Pressure & Weight Calculator

Calculation Type
Formula: Height × 0.43 psi/ft = Gravity-Fed Pressure
Height of Water Above Destination
Water Pressure Guidelines:
• 1 psi minimum for basic irrigation
• 2.5 ft height = 1 psi (minimum recommended)
• Pressure decreases as tank empties
• Longer hoses reduce pressure due to friction
• Place tanks close to usage points for better pressure

Embankment Calculator

Formulas: Bottom of Key to Top of Freeboard = Depth + 2 | Distance from Deep Zone to Center of Key = Slope Ratio × (Depth + 1 + Key Width/Slope Ratio)
Depth of Water Body
Width of Key
Width of Freeboard
Slope Ratio (horizontal:vertical)
Embankment Design Guidelines:
Key Components:
• Key: Foundation trench preventing seepage under embankment
• Freeboard: Safety margin above maximum water level
• Distance from Deep Zone: From deep zone to key centerline
Slope Ratio Guidelines:
• 2:1 (2 horizontal : 1 vertical) - Standard stable slope
• 3:1 - More conservative, better for weak soils
• 1.5:1 - Steeper slope, requires good soil conditions
• Consider soil type and local regulations
Design Considerations:
• Key depth is typically 1 meter lower than bottom of deep zone
• Freeboard prevents overtopping during storms
• Wider keys provide better seepage control
• Consider soil type for embankment stability
Construction Tips:
• Compact fill material in lifts
• Use clay core for better water retention
• Establish vegetation for erosion control
• Install spillway for overflow management

Slope Calculator

Rise
Run
Slope Guidelines:
• 0-2%: Minimal drainage, may pool water
• 2-5%: Good for rain gardens, swales
• 5-15%: Moderate erosion risk
• 15%+: High erosion risk, needs terracing

Soil Type Calculator

Sand %
Silt %
Clay %
USDA Soil Texture Classifications:
Sandy Soils:
• Sand: >85% sand, well-draining, low water retention
• Loamy Sand: 70-91% sand, moderate drainage
• Sandy Loam: Good drainage with some water retention
Loamy Soils:
• Loam: Ideal garden soil, balanced drainage and retention
• Silt Loam: High water retention, can be slow-draining
Clay Soils:
• Clay: >40% clay, very high water retention, poor drainage
• Clay Loam: Good water retention with moderate drainage
• Sandy Clay: High retention but better drainage than clay
• Silty Clay: High retention, prone to compaction
Water Retention Tips:
• Sandy soils: Add organic matter, use mulch heavily
• Clay soils: Improve drainage, avoid compaction, add compost
• Loamy soils: Maintain with organic matter and mulch
• Silt soils: Prevent erosion, improve structure with organics

Acre-Feet Calculator

Area Input Method
Formula: Area (acres) × Depth (feet) = Acre-feet
Area (acres)
Depth (feet)
Reference:
• 1 acre-foot = 325,851 gallons
• 1 hectare-meter = 2.47 acre-feet
• 1 acre-foot = 1,233,482 liters
Common Urban Lot Sizes:
• Small urban lot: 50' × 100' = 0.11 acres
• Typical suburban lot: 80' × 120' = 0.22 acres
• Large suburban lot: 100' × 150' = 0.34 acres
• 1 acre = 43,560 square feet
Calculation Tips:
• For small lots, use decimals (e.g., 0.25 acres)
• Length × Width method good for rectangular lots
• Square feet method good for irregular shapes
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Water Body Calculator – Complete Formula Reference

1. Rain Catchment Calculator

Basic Formula

Net Water Collection = Catchment Area × Rainfall × Runoff Coefficient × Conversion Factor

Area Calculations by Shape

Rectangular:

Area = Length × Width

Triangular (Base × Height):

Area = (Base × Height) ÷ 2

Triangular (Three Sides – Heron’s Formula):

s = (Side A + Side B + Side C) ÷ 2
Area = √[s × (s - Side A) × (s - Side B) × (s - Side C)]

Circular:

Area = π × radius²
Area = π × (diameter ÷ 2)²

Unit Conversions

Imperial:

Rainfall in inches → feet: rainfall ÷ 12
Volume = Area (sq ft) × Rainfall (ft) × 7.48 gal/ft³ × Runoff Coefficient

Metric:

Volume = Area (m²) × Rainfall (mm) × Runoff Coefficient = Liters

2. Cistern Sizing Calculator

Storm Event Sizing

Imperial:

Cistern Capacity = Catchment Area (sq ft) × Storm Rainfall (in) ÷ 12 × 7.48 gal/ft³ × Runoff Coefficient

Metric:

Cistern Capacity = Catchment Area (m²) × Storm Rainfall (mm) × Runoff Coefficient = Liters

Drought Period Sizing

Storage Needed = Number of People × Daily Consumption × Drought Days

3. Tank Capacity Calculator

Cylindrical Tanks

Imperial:

Volume = π × radius² × Effective Height × 7.48 gal/ft³
Volume = π × (diameter ÷ 2)² × Effective Height × 7.48 gal/ft³

Metric:

Volume = π × radius² × Effective Height ÷ 1000 = Liters
(radius and height in meters, result in cubic meters, then × 1000 for liters)

Rectangular Tanks

Imperial:

Volume = Length × Width × Effective Height × 7.48 gal/ft³

Metric:

Volume = Length × Width × Effective Height × 1000 = Liters
(dimensions in meters, result in cubic meters, then × 1000 for liters)

4. Water Pressure Calculator

Gravity-Fed Pressure

Imperial:

Pressure (psi) = Height (ft) × 0.43 psi/ft

Metric:

Pressure (kPa) = Height (m) × 9.8 kPa/m

Water Weight

Imperial:

Weight (pounds) = Volume (gallons) × 8.34 lb/gal

Metric:

Weight (kg) = Volume (liters) × 1 kg/L

5. Embankment Calculator

Key Measurements

Bottom of Key to Top of Freeboard = Water Depth + 2

Distance from Deep Zone to Center of Key = Slope Ratio × (Water Depth + 1 + Key Width ÷ Slope Ratio)

Example with 2:1 slope:

Center Distance = 2 × (Depth + 1 + Key Width ÷ 2)

6. Slope Calculator

Slope Calculations

Slope Percentage = (Rise ÷ Run) × 100

Slope Angle (degrees) = arctan(Rise ÷ Run) × (180 ÷ π)

Slope Ratio = 1 : (Run ÷ Rise)

7. Soil Type Calculator

Uses USDA Soil Texture Classification based on sand, silt, and clay percentages. Key decision points:

Primary Classifications:

  • Sand: >85% sand, silt + 1.5 × clay < 15%
  • Clay: ≥40% clay, sand ≤45%, silt <40%
  • Silt: ≥80% silt, clay <12%
  • Loam: Balanced mixture meeting specific criteria

Note: Full USDA classification uses complex decision tree with multiple criteria

8. Acre-Feet Calculator

Basic Formula

Volume (acre-feet) = Area (acres) × Depth (feet)

Area Conversion Methods

From Dimensions:

Area (acres) = (Length × Width in feet) ÷ 43,560
Area (hectares) = (Length × Width in meters) ÷ 10,000

From Square Feet/Meters:

Area (acres) = Square Feet ÷ 43,560
Area (hectares) = Square Meters ÷ 10,000

Volume Conversions

1 acre-foot = 325,851 gallons
1 acre-foot = 1,233,482 liters
1 hectare-meter = 2.47 acre-feet
1 hectare-meter = 10,000,000 liters

Key Conversion Constants

Volume Conversions

  • 1 cubic foot = 7.48 gallons
  • 1 cubic meter = 1,000 liters
  • 1 gallon = 3.78541 liters

Area Conversions

  • 1 acre = 43,560 square feet
  • 1 hectare = 10,000 square meters
  • 1 hectare = 2.47105 acres

Pressure Conversions

  • 1 foot of water = 0.43 psi
  • 1 meter of water = 9.8 kPa

Weight Conversions

  • 1 gallon of water = 8.34 pounds
  • 1 liter of water = 1 kilogram

Common Runoff Coefficients

Roofs & Impervious Surfaces

  • Metal/Pitched Roofs: 0.95
  • Concrete/Asphalt Roofs: 0.90
  • Built-up Tar & Gravel: 0.80-0.85
  • Concrete/Asphalt Pavement: 0.70-0.90
  • Gravel Surface: 0.40-0.60

Pervious Surfaces (Southwestern US)

  • Sonoran Desert (healthy): 0.30-0.50
  • Bare Earth: 0.35-0.55
  • Grass/Lawn: 0.10-0.25
  • Forest/Woods: 0.10-0.30

Knowledge of building water bodies should not be kept a secret. The math alone can be menacing. That’s why I built this calculator with resources found online as a reference for more complicated equations.

I made this calculator using Claude.ai. Please Please Please contact me if you find any errors as well as any improvements you’d like to see and I will add you to the acknowledgements! Thank you.

I’ve also converted the tool for offline use to a handy, interactive HTML file. Use the download link below

WaterBoDIY_Calculator_v0.5Download

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