Approximate Estimates: Plinth Area, Cubical Content & Unit Base Methods

Introduction

Approximate (or preliminary) estimates are used for feasibility studies, administrative approval, and quick budgets. They help decide whether a project is worth pursuing before investing in detailed design.

Three standard methods are:

1. Plinth area method
2. Cubical content method
3. Unit base method

Each uses a simple multiplier. The key is choosing the right rate from similar completed projects in your area.

Plinth Area Method

Formula

Cost of building = Plinth area × Plinth area rate

Plinth area = Length × Breadth (outer dimensions at floor level).

What to Include (IS 3861:1966)

Include:

• Area of walls at floor level
• Internal shafts (sanitary, lifts, AC ducts) not exceeding 2 sq m
• Barsati at terrace level (covered space open on one side)
• Porches of non-cantilever type

Exclude:

• Lofts
• Unenclosed balconies
• Architectural bands, cornices
• Domes, towers above terrace
• Box louvers, vertical sunbreakers

Choosing the Rate

Plinth area rate depends on:

• Quality of materials
• Type of foundation
• Height of building
• Roof type
• Woodwork and fixtures
• Number of storeys

Use rates from recent similar projects in your city. Our house construction cost calculator uses typical Indian rates (₹900–₹2,000 per sq ft) for quick estimates.

Example 1: Building Project

Given: Total plinth area = 800 sq m, rate = ₹4,500 per sq m

• Cost of building = 800 × 4,500 = ₹36,00,000
• Water supply @ 7.5% = ₹2,70,000
• Sanitary + Electrical @ 15% = ₹5,40,000
• Architectural features @ 1% = ₹36,000
• Roads & lawns @ 5% = ₹1,80,000
• P.S. & contingencies @ 4% = ₹1,44,000
  Total = ₹47,70,000
  With supervision @ 8%: ₹51,51,600

Example 2: Residential Apartment

Given: Plinth area = 500 sq m, height = 16.25 m, cubical rate = ₹1,230 per cum

• Volume = 500 × 16.25 = 8,125 cum
• Structural cost = 8,125 × 1,230 = ₹99,93,750
• Water, sanitary, electrical @ 18% = ₹17,98,875
• Architectural @ 1% = ₹99,937
• Unforeseen @ 2% = ₹1,99,875
• P.S. & contingencies @ 4% = ₹3,99,750
  Total ≈ ₹1,25,00,000

Cubical Content Method

Formula

Cost = Volume of building × Rate per unit volume

Volume = Length × Breadth × Height (out to out of walls, excluding plinth offset).

When to Use

• Multi-storeyed buildings
• When height varies significantly
• Generally more accurate than plinth area for tall structures

String course, cornice, and corbelling are usually neglected.

Example: Commercial Complex

Given:

• Plinth area per floor = 500 sq m

• Height per storey = 3.5 m

• Storeys = G+2 (3 floors)

• Cubical rate = ₹1,000 per cum

• Volume = 3 × (500 × 3.5) = 5,250 cum

• Structural cost = 5,250 × 1,000 = ₹52.5 Lakh

• Water & sanitary @ 8% = ₹4.2 L

• Electrification @ 6% = ₹3.15 L

• Fluctuation @ 5% = ₹2.625 L
  Subtotal = ₹62.475 L

• P.S. & contingencies @ 3% = ₹1.874 L

• Contractor profit @ 10% = ₹6.247 L
  Total ≈ ₹70.6 Lakh

Unit Base Method

Formula

Cost = Number of units × Rate per unit

Unit depends on building type:

• Hospital – one bed
• School/college – one student
• Hostel – one student
• Cinema – one seat

Rate = Cost of similar building ÷ Number of units.

Example 1: Hospital

50 beds, ₹60,000 per bed
Total = 50 × 60,000 = ₹30,00,000

Example 2: Hostel

150 students, ₹15,000 per student
Total = 150 × 15,000 = ₹22,50,000

Example 3: Cinema

1,700 seats, ₹6,000 per seat
Total = 1,700 × 6,000 = ₹1,02,00,000

Typical Percentage Additions

Item	Typical %
Water supply	6–12.5%
Sanitary	6–12.5%
Electrification	6–7.5%
Architectural features	1–2%
Roads & lawns	5%
Fluctuation of rates	3–5%
P.S. & contingencies	4%
Work charged establishment	1.5–2%
Contractor profit	8–10%

Adjust percentages based on project scope and local practice.

Choosing a Method

Building type	Preferred method
Residential, single/double storey	Plinth area
Multi-storey commercial/residential	Cubical content
Hospital, school, hostel	Unit base
Quick feasibility	Plinth area or unit base

Next Steps

• Detailed Estimates – Item-wise quantities and abstract
• Rate Analysis – How to derive unit rates from materials and labour