How do you calculate water sump capacity from feet and inches to litres?

First convert dimensions from feet and inches to decimal feet: Total Feet = Feet + (Inches / 12). Multiply Length × Width × Water Depth to obtain volume in cubic feet (ft³). Multiply cubic feet by 28.317 to get the total capacity in Litres (since 1 ft³ = 28.3168 Litres).

What is the standard water requirement per person per day according to Indian standards?

According to the Bureau of Indian Standards (IS 1172:1993) and the National Building Code (NBC), the minimum domestic water requirement for urban residential households with full flushing systems is 135 litres per capita per day (LPCD).

What size water sump is needed for a family of 4 to 5 members?

A standard family of 4 to 5 requires approximately 600 to 700 litres per day. For 3 to 4 days of emergency storage or tanker supply backup, a sump capacity of 5,000 to 8,000 litres is standard. In inner dimensions, a tank of 7 ft length × 5 ft width × 5 ft water depth provides approximately 5,000 litres.

What is the recommended ratio between underground sump and overhead tank?

In civil engineering practice, the underground sump is typically sized to hold 1.5 to 2 times the capacity of the overhead roof tank (OHT) to ensure uninterrupted supply during municipal water cuts.

Water Sump & Water Tank Capacity Calculator

Calculate underground water sump and overhead water tank capacity in Litres, Gallons, Cubic Feet, and Tanker loads. Supports simultaneous Feet-Inches and Metric Meter measurements.

Tank Dimensions (Inner Water Volume)

Feet & Inches

Inner Length 10 ft 0 in (3.05 m)

Inner Width 8 ft 0 in (2.44 m)

Water Depth (Effective Height) 6 ft 0 in (1.83 m)

Air Gap / Freeboard (Top Clearance) Usually 9 in (225 mm)

inches

Wall Construction Material

Total Effective Water Capacity

13,592 Litres

≈ 2.3 Commercial Tankers (6,000L loads)

US Gallons: 3,591 gal Cubic Feet: 480 ft³ Cubic Meters: 13.59 m³

Cross-Section Water Level Total Depth: 6' 9"

Freeboard Air Clearance: 9"

Water Level: 6' 0" (13,592 L)

Family Coverage

20 Days

For a 5-member family at 135 LPCD

Excavation Volume

684 ft³

Outer pit including 6" RCC walls

Required Water Volume	2,025 Litres
Recommended Inner Length	5' 6" (1.68 m)
Recommended Inner Width	4' 0" (1.22 m)
Recommended Water Depth	4' 6" (1.37 m)
Total Sump Depth (incl. 9" freeboard)	5' 3" (1.60 m)

Complete Engineering Guide: Underground Water Sump & Water Tank Sizing

An Underground Water Sump is a subterranean reservoir constructed from reinforced cement concrete (RCC) or brick masonry to store water from municipal piped supplies (such as Cauvery, Jal Board, BWSSB, or DJB) or borewells. Because water supply in most Indian urban agglomerations is intermittent, an accurately sized sump is mandatory to ensure uninterrupted domestic pressure and hygiene.

1. National Building Code (NBC) Water Consumption Norms

Under the Bureau of Indian Standards code IS 1172:1993, domestic water requirements per capita are standardized as follows:

135 Litres per Capita per Day (LPCD): Standard urban residential dwelling with full flushing system (Drinking: 5L, Cooking: 5L, Bathing: 55L, Flushing: 30L, Washing clothes: 20L, Washing utensils: 10L, House cleaning: 10L).
200 LPCD: High-end luxury apartments and independent villas with bathtubs, private lawns, and multi-vehicle washing.
45 LPCD: Rural communities or low-income housing with standalone common standposts.

2. Sizing Formula: Water Sump vs. Overhead Tank (OHT)

A sound civil engineering rule of thumb dictates that the underground sump capacity should equal 1.5 to 2.0 times the daily domestic demand, while the overhead roof tank (OHT) should equal approximately 0.5 times (half day) to 1 day of demand. This distribution ensures the rooftop structure does not endure excessive static dead load while the underground sump buffers against 2 to 3 days of supply failure.

Capacity (Litres) = Length (ft) × Width (ft) × Water Depth (ft) × 28.3168

3. Why Freeboard Clearance is Critical

Never fill a sump up to the top structural slab. A mandatory freeboard clearance of 225 mm to 300 mm (9 to 12 inches) must be maintained between the maximum high-water level and the bottom of the roof slab. Freeboard provides:

Operating clearance for the mechanical float ball valve.
Air circulation to prevent foul smells and stagnant water gases.
Safety overflow space to prevent backflow into municipal supply pipelines.

4. RCC vs. Brick Masonry: Which is Best for Sumps?

RCC (Reinforced Cement Concrete): Best for clayey soils, high water tables, and parking areas carrying vehicular loads. 150 mm (6-inch) M20/M25 concrete walls with Dr. Fixit or integral waterproofing admixtures eliminate leakage risk.
Brick Masonry: Suitable only for low water table rocky terrain and smaller capacities (<5,000 Litres). Requires 230 mm (9-inch) thick walls with double-coat plastering and chicken mesh to resist outward hydrostatic water pressure.