Plinth area calculator for building estimates
This plinth area calculator works out the plinth area — the built-up covered area of a building — floor by floor from its dimensions, the way an engineer measures it for an estimate. Enter each floor’s length and width to the outside faces of the walls, or inside faces plus the wall thickness, add balconies, verandahs and alcoves, and it applies the counting rules of IS 3861:2002.
It then prices the building at a plinth area rate: area × rate, adjusted by a cost index for the place, plus services and escalation. That is how CPWD-style plinth area rates are used for preliminary estimates. The rate is yours to enter; no published rate is built in.
Buying a flat rather than building? Carpet, built-up and super built-up conversions with loading belong in the carpet area calculator; this page is the construction view.
How to use the Plinth Area Calculator
- Choose metres or feet and whether your lengths are to the outer faces of the walls or the inside faces. For inside faces, enter the external wall thickness.
- Add one row per floor block: give it a floor name, choose “Enclosed floor”, and enter length and width. Rows with the same floor name are added together.
- Add balconies, verandahs, alcoves, courtyards and separately measured parts such as a stair cover (mumty) or porch, choosing the matching type.
- Optionally enter the internal wall length per floor to see the area net of walls.
- Enter the plinth-area rate, its source and year, the cost index for your place, services and escalation, then copy the summary or download a CSV.
Plinth Area Calculator features
IS 3861 counting rules
Covered balconies and verandahs 100%, open ones 50%, alcoves 25/50/100% by height, cut-outs deducted.
Wall thickness built in
Inside dimensions become outer dimensions by adding twice the wall thickness, and wall area is estimated.
Per-floor table
Each floor’s parts, plinth area and approximate area net of walls, with a building total.
Separate categories
Stilt floors, garages, porches and mumtys are kept out of the floor total, as IS 3861 asks.
Plinth-area-rate cost
Rate per m² or sq ft × cost index, plus services and escalation, in ₹, $, £ or AED.
Built-up area calculation formula
For a rectangular floor measured to the outer faces of the walls, plinth (built-up) area = outer length × outer width. If you measured inside the rooms, add the wall thickness on both sides first: (inner length + 2t) × (inner width + 2t). Worked example: a 12 × 9 m house with 230 mm walls has 108 m² of plinth area per floor. Two floors, a 3 × 1.2 m covered balcony and a 2 × 1 m open balcony give 108 + 108 + 3.6 + 1.0 = 220.6 m² (2,375 sq ft); the 3 × 3 m stair cover on the terrace is measured separately.
The external walls take (2 × (12 + 9) − 4 × 0.23) × 0.23 = 9.45 m² of each floor, and 25 m of 115 mm internal walls another 2.88 m², leaving about 95.7 m² inside the walls on the ground floor.
What IS 3861 counts in plinth area
Lengths are measured to the nearest 0.01 m and areas to 0.01 m² (cl. 3.1). The standard does not recognise “super built-up” area.
| Item | Counted |
|---|---|
| Walls at floor level (not plinth offsets), shafts, lift wells, staircase | 100% |
| Balcony or verandah with parapet, covered by a projection above | 100% |
| Balcony or verandah, unprotected from above | 50% |
| Cantilevered alcove up to 1 m / 1–2 m / over 2 m high | 25% / 50% / 100% |
| Common wall owned jointly with a neighbour | half the wall |
| Loft, cornice and bands, sunshades, open platform, terrace, open spiral stair | not counted |
| Basement, stilt floor, mezzanine, garage, porch, mumty, machine room | measured separately |
Plinth area vs carpet area, and CPWD plinth area rates
Plinth area includes the walls; carpet area is the usable floor inside them, and IS 3861 also leaves verandahs, corridors, stairs, kitchens, baths and stores out of carpet area. RERA defines carpet area differently for flat sales. As a rough guide, walls take 10–15% of a house’s plinth area.
CPWD publishes Plinth Area Rates for its building types (the 2023 edition is the one Indian Railways adopted in a July 2025 letter) with a building cost index for other places. Estimates multiply the plinth area by the rate and by the place’s index, then add services such as water supply, sanitary and electrical work. Use the edition and index your department specifies. The house construction cost calculator gives a market-rate budget, and the labour cost calculator prices the labour by trade.
Plinth Area Calculator FAQ
How do I calculate plinth area?
Multiply the outer length by the outer width of each floor, including the walls, then add 100% of covered balconies, 50% of open balconies and the IS 3861 share of alcoves. Add floors together; keep stilt floors, porches and mumtys separate.
Is plinth area the same as built-up area?
In practice yes: IS 3861 defines plinth area as the built-up covered area at floor level. Builders’ “built-up area” usually means the same thing — carpet area plus walls, plus balconies by their own rule.
What is the difference between plinth area and carpet area?
Plinth area includes the walls and covered spaces; carpet area is the usable room area inside the walls. Carpet area is typically 80–90% of plinth area for a house.
How do I use a CPWD plinth area rate?
Estimated cost = plinth area × rate × (cost index of the place ÷ 100), plus the extra items listed for services. Enter all three in this calculator; take the rate and index from the edition your department uses.
Should I measure to the inside or outside of the walls?
Plinth area is to the outside faces. If your drawing gives room sizes, choose “Inside faces” and enter the wall thickness, and the calculator adds it on both sides.
Is the cost a quote?
No. It is a preliminary estimate from the rate you enter. Detailed estimates come from quantities of each item, not plinth area alone.