Soil Moisture Calculator

Determine your soil’s volumetric water percentage, plant-available water holding capacity, and the exact gallons or inches of irrigation required to recharge your root zone to field capacity.

Method reviewed by Plant Calculator Team·Updated August 2026

How this is worked out

θg = (Wet Mass - Dry Mass) ÷ Dry Mass × 100  ·  θv = θg × Bulk Density  ·  Water Deficit (in) = (Field Capacity % - Current θv %) × Root Depth

Calculates both laboratory oven-dry core samples and field TDR/capacitance sensor moisture readings against soil texture hydraulic limits.

Assumptions

Field Capacity (FC) is the water content retained after gravitational water has drained (typically 24–48 hours after saturating rain).

Permanent Wilting Point (PWP) is the soil water potential (-15 bars / -1500 kPa) where plant roots can no longer extract capillary moisture.

Management Allowed Depletion (MAD) threshold is typically 50% for vegetables and turf before water stress reduces photosynthetic yield.

Soil dry bulk density typically ranges from 1.15 g/cm³ in high-organic loams to 1.60 g/cm³ in compacted sands.

Soil Water Physics & Irrigation Management

Soil water content is the single most critical factor regulating plant transpiration, nutrient transport, microbial respiration, and crop yield. Soil acts as a porous sponge: pores hold water through capillary adhesion and cohesion against the downward pull of gravity. Understanding the physical boundaries of soil moisture allows growers to irrigate with precision—eliminating drought stress while preventing the root hypoxia and nitrogen leaching caused by over-watering.

The Three Physical Soil Water States

1. Saturation (0 kPa / 0 bar): Every pore space (macro and micro) is 100% filled with water. Air is excluded. Gravitational water drains away downward over 24–48 hours. Prolonged saturation causes root suffocation and root rot pathogens (Pythium, Phytophthora).
2. Field Capacity (FC | -10 to -33 kPa / -0.1 to -0.33 bar): The ideal moisture balance. Gravitational drainage has ceased. Micro-pores are filled with capillary water while macro-pores are filled with oxygen. Plant roots absorb moisture with zero energy resistance.
3. Permanent Wilting Point (PWP | -1500 kPa / -15 bar): Only a microscopic film of water remains tightly bound to clay and mineral surfaces. Plant root suction cannot overcome this tension; leaves wilt permanently and cannot recover even if placed in a humid chamber.

Soil Texture Water Holding Capacity Table

Hydraulic properties by soil texture class
Soil Texture Class	Field Capacity (θv %)	Wilting Point (θv %)	Available Water (AWC in/ft)	Bulk Density (g/cm³)
Sand	10%	4%	0.72 in/ft (60 mm/m)	1.55–1.65
Loamy Sand	14%	6%	0.96 in/ft (80 mm/m)	1.50–1.60
Sandy Loam	20%	9%	1.32 in/ft (110 mm/m)	1.40–1.50
Loam	28%	13%	1.80 in/ft (150 mm/m)	1.30–1.40
Silt Loam	32%	14%	2.16 in/ft (180 mm/m)	1.25–1.35
Clay Loam	34%	18%	1.92 in/ft (160 mm/m)	1.20–1.30
Heavy Clay	38%	24%	1.68 in/ft (140 mm/m)	1.15–1.25

Understanding Management Allowed Depletion (MAD)

Plants should never be forced to extract water all the way to the Permanent Wilting Point. As soil dries past the halfway mark, capillary tension increases exponentially, forcing roots to expend metabolic energy sucking water rather than producing fruit. The Management Allowed Depletion (MAD) threshold is the point at which irrigation should be triggered:

40% MAD (Shallow-rooted & Sensitive Crops): Strawberries, onions, celery, shallow lettuce. Irrigate frequently with lighter doses.
50% MAD (Standard Agronomic Rule): Tomatoes, peppers, sweet corn, brassicas, turfgrass, fruit trees.
60% MAD (Deep Rooted & Drought Hardened): Grapes, olives, cotton, mature alfalfa. Deep, infrequent waterings encourage deep taproot expansion.

Three Worked Examples

1. Vegetable Garden (1,000 sq ft) on Loam Soil with 12-inch Root Zone

Loam Field Capacity = 28% θv. Permanent Wilting Point = 13% θv. Total Available Water (TAW) = (0.28 - 0.13) × 12 in = 1.80 inches of water. If a capacitance sensor reads current moisture at 18% θv, the water deficit is (0.28 - 0.18) × 12 in = 1.20 inches of water. For 1,000 sq ft, 1.20 inches equals 748 gallons of irrigation.

2. Lawn Turf (5,000 sq ft) on Sandy Loam (6-inch Root Zone)

Sandy Loam FC = 20%, PWP = 9%. TAW = (0.20 - 0.09) × 6 in = 0.66 inches. Trigger 50% MAD threshold = 20% - (0.50 × 11%) = 14.5% θv. When moisture drops to 14.5%, apply (0.20 - 0.145) × 6 in = 0.33 inches of water (0.33 × 0.623 × 5000 = 1,028 gallons).

3. Laboratory Oven-Dry Core Test

Soil core volume = 100 cm³. Wet mass = 150 g. Oven-dry mass = 125 g. Gravimetric moisture θg = (150 - 125) ÷ 125 = 20.0%. Dry bulk density ρb = 125 g ÷ 100 cm³ = 1.25 g/cm³. Volumetric moisture θv = 20.0% × 1.25 = 25.0% θv.

Natural Moisture Conservation: Living Mulches & Organic Blankets

While precision irrigation calculates exact refill deficits, protecting the soil surface with organic mulch dramatically lowers overall irrigation frequency. Applying a 2 to 3-inch blanket of arborist wood chips, shredded leaves, or clean straw cuts direct solar evaporation by 60% to 75%, maintains soil temperatures up to 15°F cooler in midsummer heat, and prevents crusting so every drop of rain or irrigation infiltrates freely into root zones.

Common questions

What is the difference between Gravimetric (θg) and Volumetric (θv) soil moisture?

Gravimetric soil water content (θg) is the ratio of water mass to dry soil mass (grams of water per gram of dry soil). Volumetric water content (θv) is the volume of water per unit volume of total soil (cm³ of water / cm³ of soil). Because irrigation is measured in depth (inches or mm), volumetric water content (θv = θg × Bulk Density) is the universal standard for irrigation scheduling.

What is Field Capacity vs. Permanent Wilting Point?

Field Capacity (FC, typically -0.33 bar tension) is the maximum amount of water a soil can hold against gravity. Permanent Wilting Point (PWP, -15 bar tension) is the minimum water level below which plant roots cannot exert enough suction to extract water, causing irreversible leaf wilting. The difference between FC and PWP is the Total Available Water (TAW).

How do I perform a gravimetric oven-dry test at home?

Collect a representative undisturbed soil core using a metal ring of known volume. Weigh the fresh soil sample immediately (M_wet). Dry the sample in a kitchen oven at 220°F (105°C) for 16–24 hours until its weight stops changing, then record the dry weight (M_dry). Gravimetric moisture % is ((M_wet - M_dry) / M_dry) × 100.

How much water should I apply when soil reaches 50% MAD?

When current volumetric moisture drops below the 50% Management Allowed Depletion threshold (halfway between Field Capacity and Wilting Point), apply exactly enough water to return the active root zone to Field Capacity (usually 0.75 to 1.5 inches of water). Applying more creates runoff and leaches nitrate nutrients below roots.

Soil Moisture & Irrigation Deficit Inputs