Method of Measurement of Plinth, Carpet and Rentable Area of Buildings

IS 3861 - Scope Summary & Key Points

Scope (Clause 2.0):
Defines the application and definitions relevant to the standard. It governs measurement methods for civil engineering works (excluding river valley projects).

Key Specifications:

• Rounding Off:
  Final test or analysis values must be rounded per IS 2:1960 rules, keeping the same significant figures as specified.

• Rentable Area Calculation (Clause 6.2.2):
  For non-residential buildings, rentable area = carpet area + 1/4 carpet area.

Important Notes:

Summary Formula:

Rentable Area = Carpet Area + (1/4 × Carpet Area) = 1.25 × Carpet Area

If you need detailed measurement methods or specific tables, refer to the full IS 3861 document or related sections in IS 1200 series for civil works measurement.

IS 3861 - Definitions: Key Points

• Clause 2.0: Defines terms used in the standard to ensure uniform understanding.

• Clause 6.2.2 (Table 6.2): For Non-Residential Buildings (category 3.2 b)), the rentable area is calculated by adding one-fourth of the carpet area.

Important Specification:

Notes:

• Carpet area: Usable floor area within the walls.
• Rentable area includes usable area plus a fraction of common/shared spaces.
• The standard follows IS 2:1960 for rounding off numerical values.

This ensures consistent measurement in civil engineering works, especially for billing and planning.

flowchart LR
    A[Carpet Area] --> B[Add 1/4 Carpet Area]
    B --> C[Rentable Area]

For detailed definitions, refer to Clause 2.0 of IS 3861.

IS 3861: Categories of Areas to be Measured Separately

Key Categories (Clause 3.2) to Measure Separately:

• b) Floor without cladding (stilted floor)
• e) [Not specified in provided context]
• g) Stair cover (mumty)
• h) Machine room
• k) Towers, turrets, domes projecting above terrace level at terrace
• m) [Not specified in provided context]

Important Notes:

• Carpet areas of categories b, e, g, h, k, m are NOT required to be calculated (Clause 5.2.1).
• For category b) (floor without cladding), only one-fourth of carpet area is accounted in rentable area (Clause 6.2.2).

Measurement Precision (Clause 3.1):

• Linear measurements to nearest 0.01 m
• Areas to nearest 0.01 m²

Summary Table for Rentable Area Accounting (Clause 6.2.2):

Practical Implication:

• Measure these categories separately.
• Exclude or apply factor (¼) as per category for rentable area calculations.

flowchart TD
    A[Start Measurement] --> B{Category}
    B -->|b) Floor without cladding| C[Measure Carpet Area]
    C --> D[Account 1/4 Carpet Area]
    B -->|g), h), k), m)| E[Exclude Area from Carpet Calculation]
    B -->|Others| F[Calculate Carpet Area Normally]

This ensures clear segregation and correct rentable area computation as per IS 3861.

IS 3861: Measurement of Plinth Area – Key Points

Definition (Clause 2.1)

• Plinth Area: Built-up covered area measured at the floor level of basement or any storey.

Separate Measurement (Clause 3.2)

Measure separately (do not combine):

• Floors without cladding (stilted floors)
• Partly covered top floors
• Mezzanine floors (e.g., assembly seating)
• Service staff accommodation
• Stair covers (mumty)
• Machine rooms
• Towers, turrets, domes above terrace level

Inclusion in Plinth Area (Clause 4.1.1)

Notes

• For common walls owned jointly, include only half the wall area in plinth area.
• Corrugated sheet cladding: measure to outer edge of corrugation.

Summary Formula for Plinth Area (A_p):

[ A_p = A_{\text{walls}} + A_{\text{shafts}} + A_{\text{verandah}} + A_{\text{balcony}} + A_{\text{alcove}} ]

Where each component is adjusted by the % inclusion as above.

flowchart TD
    A[Start: Floor Level] --> B{Measure Wall Area}
    B --> C[Include 100% (external cladding face)]
    C --> D{Measure Shafts}
    D --> E[Include 100%]
    E --> F{Measure Verandah}
    F --> G[Protected: 100%]
    F --> H[Unprotected: 50%]
    G & H --> I{Measure Balcony}
    I --> J[Protected: 100%]
    I --> K[Unprotected: 50%]
    J & K -->

IS 3861: Measurement of Carpet Area – Key Points

Definition of Carpet Area (Clause 5.2)

Carpet area = Built-up area excluding the following:

• Corridor and passage
• Entrance hall and porch
• Staircase and stair-cover (mumty)
• Lift shaft and machine room
• Bathroom and lavatory
• Kitchen and pantry
• Air-conditioning duct and plant room
• Shafts for sanitary, water supply, garbage chute, electrical, fire fighting, telecom, lift

Note: In halls/basements, area 1 m beyond last step is included as staircase.

Formula for Carpet Area

[ \text{Carpet Area} = \text{Plinth Area} - \sum \text{(Excluded areas as above)} ]

Specifications:

• Carpet area excludes service and circulation spaces.
• Carpet area is the usable floor area within the walls.
• Categories in 3.2 (b, e, g, h, k, m) are not required to be calculated for carpet area.

Summary Table of Excluded Areas

flowchart TD
    A[Plinth Area] --> B[Subtract Corridors, Passages]
    B --> C[Subtract Entrance Hall, Porch]
    C --> D[Subtract Staircase & Mumty]
    D --> E[Subtract Lift Shaft & Machine Room]
    E --> F[Subtract Bathroom & Lavatory]
    F --> G[Subtract Kitchen & Pantry]
    G --> H[Subtract AC Duct & Plant Room]
    H --> I[Subtract Sanitary/Water Supply Shafts]
    I --> J[Carpet Area (Usable Floor Area)]

This method ensures uniformity in carpet area measurement as per IS 3861.

IS 3861: Measurement of Rentable Area - Key Points

Definition (Clause 2.3)

• Rentable Area = Carpet area at any floor level including additional areas as per Clause 6.

Non-Residential Buildings (Clause 6.2)

• Rentable Area = Carpet area (Clause 5) + Carpet area of canteen (including store, kitchen, pantry attached).

Special Accounting (Clause 6.2.2)

• For category in 3.2 b), only 1/4th of carpet area is accounted in rentable area.

Carpet Area Exclusions (Clause 5.2.1)

• Carpet areas of categories 3.2 b), e), g), h), k), m) not required to be calculated.

Summary Table for Rentable Area Calculation

Practical Formula

[ \text{Rentable Area} = \text{Carpet Area} + \text{Canteen Area} + \frac{1}{4} \times \text{Category 3.2 b) Carpet Area} ]

This ensures consistent measurement of rentable space per IS 3861 for cost estimation and billing.

flowchart TD
    A[Start: Carpet Area Calculation] --> B{Building Type?}
    B -->|Non-Residential| C[Add Canteen Area]
    B -->|Category 3.2 b)| D[Add 1/4th Carpet Area]
    B -->|Others (3.2 e,g,h,k,m)| E[Exclude Carpet Area]
    C --> F[Calculate Rentable Area]
    D --> F
    E --> F
    F --> G[End]

For detailed measurement methods, refer to Clause 5 & 6 of IS 3861.

IS 3861 - Rounding Off Rules Summary

Key Clause:

• Final test or calculated values must be rounded off as per IS 2:1960 - Rules for rounding off numerical values.
• The number of significant figures in the rounded value should match the specified value in the standard.

IS 2:1960 Rounding Off Essentials

• Retain the same decimal places as the specified value.
• Example: If specified value is 12.345 (3 decimal places), round off results to 3 decimal places.

IS 3861 Specifics on Measurement (Clause 3.1)

• Linear measurements: Round to nearest 0.01 m
• Area measurements: Round to nearest 0.01 m²

Practical Application

• If a length = 5.6789 m, round to 5.68 m
• If area = 12.3456 m², round to 12.35 m²

flowchart TD
    A[Measured Value] --> B{Check Digit after last retained}
    B -->|<5| C[Keep last digit unchanged]
    B -->|≥5| D[Increase last digit by 1]
    C --> E[Rounded Value]
    D --> E

Summary: Always round off results per IS 2:1960, matching decimal places of the standard's specified value, ensuring consistency and compliance in measurements.

IS 3861: Special Provisions for Mezzanine Floors

Key Points from IS 3861:

• Clause 2.5: Defines mezzanine floors as intermediate floors with minimum clear height of 1.8 m (where local rules permit).
• Clause 1.8 (Note): Intermediate floors with at least 1.8 m clear height qualify as mezzanine floors for measurement.
• Clause 6.2.2 (Table 6.2): For non-residential buildings, only one-fourth (25%) of the mezzanine carpet area is accounted as rentable area.

Summary Table for Mezzanine Floor Area Accounting:

Specifications:

• Minimum clear height: 1.8 m (for classification as mezzanine).
• Mezzanine floors are measured separately and not combined with other floor areas (Clause 3.2 d).
• Used typically in assembly buildings, theatres, auditoriums for additional seating.

Practical Notes:

• When designing mezzanine floors, ensure structural support accommodates live loads typical of occupancy.
• Follow local building codes for fire safety, egress, and ventilation specific to mezzanine floors.
• Mezzanine floor loadings often range from 2 to 5 kN/m² depending on usage.

flowchart TD
    A[Start: Local Rules Permit Intermediate Floor] --> B{Clear Height ≥ 1.8 m?}
    B -- Yes --> C[Classify as Mezzanine Floor]
    B -- No --> D[Not Mezzanine Floor]
    C --> E[Measure Carpet Area]
    E --> F[Account 25% Carpet Area for Rentable Area (Non-residential)]

References:

• IS 3861 Clause 2.5, 1.8, 3.2(d), 6.2.2 Table 6.2
• Local building regulations for mezzanine floor clearance and use.

IS 3861: Inclusions & Exclusions in Area Calculations

Plinth Area (Clause 4.1.1 & 4.1.2)

Included:

• Wall area at floor level (excluding plinth offsets); for columns beyond cladding, up to external cladding face.
• Shafts for sanitary, water supply, garbage chute, telecom, electrical, fire fighting, A/C, lifts.
• Open verandah:
  • 100% area if protected by projection above.
  • 50% area if unprotected.
• Balcony:
  • 100% if protected.
  • 50% if unprotected.
• Alcoves (cantilever slabs beyond wall):
  • 25% area if height ≤ 1 m.
  • 50% area if height > 1 m and ≤ 2 m.
  • 100% area if height > 2 m.

Excluded:

• Loft area.
• Architectural bands, cornices.
• Vertical sun breakers, box louvers, flower pot slabs.
• Open platforms.
• Open spiral/service staircases.
• Mumty, machine rooms, towers, turrets, domes above terrace.

Carpet Area (Clause 5.1.2)

Exclusions from wall area:

• Pilasters ≤ 300 cm².
• Chullah platforms projecting beyond wall face.

Summary Table

flowchart TD
    A[Plinth Area]

IS 3861: Treatment of Shared Walls and Joint Ownership

Key Points on Shared Walls (from context & IS provisions):

• Measurement of Shared Walls for Plinth Area:
  When walls are owned jointly by different owners, the area of the shared wall should be divided equally between the owners for plinth area calculation.

• Wall Area Inclusion (Clause 5.1.1):
  Include in wall area:

  • Doors and openings
  • Pillars, intermediate pillars, supports within plinth area
  • Pilasters along walls exceeding 300 cm²
  • Flues inside the wall

• Wall Area Exclusion (Clause 5.1.2):
  Exclude from wall area:

  • Pilasters not exceeding 300 cm²
  • Chullah platforms projecting beyond wall face

Formula for Shared Wall Area Allocation:

[ \text{Area credited to each owner} = \frac{\text{Total shared wall area}}{2} ]

Practical Notes:

• For residential buildings (Clause 6.1), this shared wall treatment applies.
• Ensure precise measurement of wall thickness and height for accurate plinth area.
• Pilaster area threshold (300 cm²) determines inclusion/exclusion.

flowchart LR
    A[Shared Wall] --> B[Measure Total Wall Area]
    B --> C[Divide Area Equally]
    C --> D[Owner 1 Area]
    C --> E[Owner 2 Area]

Summary:
For joint ownership, divide the shared wall area equally. Include doors, large pilasters (>300 cm²), pillars, and flues in wall area; exclude small pilasters and projecting platforms. This ensures fair plinth area measurement per IS 3861.

IS 3861: Measurement of Service Areas and Shafts

Key Points:

• Internal Shafts/Ducts:
  Areas of internal shafts/ducts (service shafts, ducts, etc.) are exhaustively detailed for inclusion in plinth area calculations. Their areas must be measured separately and included as per the code.

• Measurement Precision:

  • Linear measurements: nearest 0.01 m
  • Areas: nearest 0.01 m² (Clause 3.1)

• Area Categories:
  Service areas like shafts, ducts, machine rooms, stair covers, etc. are measured separately (Clause 3.2). They are not clubbed with other floor areas.

• Rentable Area (Non-Residential Buildings):
  Per Table 6.2.2, one-fourth of the carpet area of certain categories (Clause 3.2 b) is accounted for as rentable area.

Typical Formula for Plinth Area Including Shafts/Ducts

[ \text{Plinth Area} = \sum (\text{Floor Area}) + \sum (\text{Service Shafts/Ducts Area}) ]

• Measure shaft/duct cross-section area at floor level.
• Include area in plinth area calculation.
• For rentable area, apply factor (e.g., 1/4 carpet area as per Clause 6.2.2).

Summary Table: Measurement Precision

Conceptual Diagram of Area Inclusion

flowchart TD
    A[Floor Area] --> C[Plinth Area]
    B[Service Shafts/Ducts Area] --> C
    C --> D[Total Plinth Area]

Note: For detailed shaft types and their inclusion criteria, refer to the exhaustive classification in IS 3861 clauses on internal shafts/ducts.

IS 3861: Measurement of Stair Covers (Mumty)

Definition (Clause 2.6)

• Stair Cover (Mumty): A roofed structure over stairs and landing, enclosing only the staircase for weather protection, not for habitation.

Measurement Categories (Clause 3.2)

• Stair cover (mumty) area must be measured separately and not combined with other floor areas.

Carpet Area Exclusion (Clause 5.2)

• Stair cover (mumty) area is excluded from carpet area calculations.

Key Points for Measurement:

Practical Formula for Stair Cover Area:

[ \text{Area}{mumty} = \text{Length}{stairs} \times \text{Width}_{stairs} ]

• Length & width measured at the external dimensions of the stair cover roof.

flowchart TD
    A[Staircase & Landing] --> B[Covered by Mumty]
    B --> C[Measured Separately]
    C --> D[Excluded from Carpet Area]
    C --> E[Included in Plinth Area]

Summary: Measure stair covers (mumty) as a separate roofed area enclosing stairs only, exclude from carpet area, and include up to 1 m beyond last step. Use external dimensions for area calculation.

IS 3861 - Committee Composition Summary

• The Committee responsible for IS 3861 is the Methods of Measurement of Works of Civil Engineering (Excluding Water Resources Development Division) Sectional Committee, CED 44.
• The composition is detailed in Annex A of the standard.
• It includes representatives from:
  • Personal capacities (e.g., Shri O. P. Goel - Chairman)
  • Government bodies (CPWD, Central Water Commission, Public Works Departments)
  • Industry (Builders Association of India, Gammon India, Hindustan Steel Works)
  • Research and consultancy (Engineers India Limited, M.N. Dastur & Co.)
  • Professional institutions (Indian Roads Congress, Institution of Surveyors)
  • BIS Directorate General (Ex-officio member)
• The Member-Secretary is Shri R. K. Gupta, Joint Director (Civil Engineering), BIS.

This committee ensures the standard's technical accuracy and relevance by involving multidisciplinary experts from government, industry, and academia.

graph TD
    A[IS 3861 Committee] --> B[Chairman: Shri O. P. Goel]
    A --> C[Government Bodies]
    A --> D[Industry Representatives]
    A --> E[Professional Institutions]
    A --> F[BIS Directorate General]
    C --> CPWD
    C --> CWC
    C --> PWD
    D --> Builders_Association
    D --> Gammon_India
    D --> Hindustan_Steel
    E --> IRC
    E --> Institution_of_Surveyors
    F --> BIS_Head

Note: For detailed names and roles, refer to Annex A of IS 3861.