Estimates in civil engineering fall into two main groups: approximate (preliminary) estimates such as the plinth area, cubic content, unit base and typical bay methods, and the detailed estimate, which measures every item from the drawings. Revised, supplementary and maintenance estimates are prepared later in a project's life. For detailed building estimates, quantities are measured by the long wall–short wall method or the centre line method.
Below, each type of estimate is explained briefly, followed by a solved centre line example for a one-room building that you can use for exams and real work.
Approximate or preliminary estimates
These give a quick idea of cost for budgeting, feasibility and administrative approval, before full drawings exist.
Plinth area method
Cost = plinth area × rate per m² of a similar building recently built in the same area. Plinth area is the covered area at floor level measured to the outside of walls. A building 9 m × 12 m has a plinth area of 108 m², about 1,162.5 sq ft. Multiply by a current local rate, not an old one. Our house construction cost estimator works on this idea.
Cubic content method
Cost = plinth area × height × rate per m³. Height is usually taken from floor level to top of roof, leaving out foundations and parapets. For the same building with 3.2 m storey height: 108 × 3.2 = 345.6 m³ per storey. This method suits buildings where height varies, such as halls and warehouses.
Unit base method
Cost is worked out per unit of use: per bed for a hospital, per student for a school or hostel, per seat for a hall, per km for a road. It is useful at the planning stage when only capacity is known.
Typical bay method
For buildings made of repeating bays, such as factory sheds or long hostel blocks, one typical bay is estimated in detail and multiplied by the number of bays. End bays and any special parts are added separately.
Detailed, revised, supplementary and maintenance estimates
Detailed estimate: every item is measured from working drawings (length × breadth × height), priced from a rate analysis, and summed in an abstract with contingencies and VAT. This is the estimate that gets technical sanction and becomes the source of the bill of quantities.
Revised estimate: a fresh detailed estimate prepared when the sanctioned amount is likely to be exceeded beyond the limit allowed by the rules, or when the design changes a lot. It includes a comparison with the original.
Supplementary estimate: covers extra work added during construction that was not in the original scope, such as a boundary wall added later. It shows only the additional work.
Maintenance (annual repair) estimate: prepared each year to keep a building or road in service: painting, minor plaster repair, cleaning drains, patching roads.
| Type | When it is made | Basis | Accuracy |
|---|---|---|---|
| Plinth area | Planning | Rate per m² of floor | Rough |
| Cubic content | Planning | Rate per m³ of volume | Rough |
| Unit base | Planning | Rate per bed, seat, student, km | Rough |
| Typical bay | Planning | One bay in detail × number of bays | Fair |
| Detailed | Before tender | Full measurement and rate analysis | Highest |
| Revised | During work, if cost overrun | Fresh detailed estimate | High |
| Supplementary | During work, extra scope | Detailed estimate of extra items | High |
| Maintenance | Every year | Repair items measured | Fair to high |
Methods of building estimation
Long wall–short wall method
Walls in one direction are taken as long walls, measured out-to-out. Walls in the other direction are short walls, measured in-to-in. For each layer of the foundation:
- Long wall length = centre-to-centre length + width of that layer
- Short wall length = centre-to-centre length − width of that layer
The width changes for each step of the footing, so every layer needs its own lengths. It is slow but easy to check.
Centre line method
The total length of all wall centre lines is found once and multiplied by the cross-section area of each layer. Where walls meet in a T-junction, deduct half the layer width once for every junction. Corners of a closed rectangle need no deduction. The method is fast and works best when all walls have the same section.
Worked example: centre line method for a one-room building
Data: internal size 4.00 m × 3.00 m, brick walls 0.23 m thick.
- Foundation trench 0.90 m wide × 1.00 m deep
- PCC bed 0.90 m wide × 0.15 m thick
- Brick footing: first step 0.69 m × 0.25 m, second step 0.46 m × 0.25 m, then 0.23 m wall × 0.35 m up to ground level
- Plinth wall 0.23 m × 0.45 m above ground
- Superstructure wall 0.23 m × 3.00 m, one door 1.0 × 2.1 m, two windows 1.2 × 1.2 m
Centre line length: centre-to-centre lengths are 4.00 + 0.23 = 4.23 m and 3.00 + 0.23 = 3.23 m. Total = 2 × (4.23 + 3.23) = 14.92 m. There are only four corners and no T-junctions, so no deduction is needed.
| Item | Length (m) | Breadth (m) | Height (m) | Quantity (m³) |
|---|---|---|---|---|
| Earthwork in excavation | 14.92 | 0.90 | 1.00 | 13.43 |
| PCC in foundation | 14.92 | 0.90 | 0.15 | 2.01 |
| Brickwork, first footing step | 14.92 | 0.69 | 0.25 | 2.57 |
| Brickwork, second footing step | 14.92 | 0.46 | 0.25 | 1.72 |
| Brickwork, wall to ground level | 14.92 | 0.23 | 0.35 | 1.20 |
| Brickwork in plinth | 14.92 | 0.23 | 0.45 | 1.54 |
| Brickwork in superstructure | 14.92 | 0.23 | 3.00 | 10.29 |
| Deduct door | 1.00 | 0.23 | 2.10 | −0.48 |
| Deduct two windows | 2 × 1.20 | 0.23 | 1.20 | −0.66 |
| Net brickwork in superstructure | 9.15 | |||
Total brickwork in foundation is 2.57 + 1.72 + 1.20 = 5.49 m³, and all brickwork together is 5.49 + 1.54 + 9.15 = 16.18 m³. In a full estimate you would also deduct lintel and any RCC band volumes from the brickwork. To turn brickwork volume into number of bricks, cement and sand, use the brick calculator.
Check with the long wall–short wall method
For excavation (width 0.90 m):
- Long walls: 4.23 + 0.90 = 5.13 m each; 2 × 5.13 × 0.90 × 1.00 = 9.234 m³
- Short walls: 3.23 − 0.90 = 2.33 m each; 2 × 2.33 × 0.90 × 1.00 = 4.194 m³
- Total = 13.428 m³, the same as the centre line result
If an inner partition wall were added, it would create two T-junctions. Its centre line length would be added to the total, and for excavation you would deduct 2 × 0.90 / 2 = 0.90 m of length. You can cross-check trench volumes with the excavation calculator.
Frequently asked questions
Which type of estimate is the most accurate?
The detailed estimate, because every item is measured from working drawings and priced by rate analysis. Approximate methods are for early budgeting only.
What is the difference between plinth area and carpet area?
Plinth area includes the walls; carpet area is the usable floor inside them. In the example, plinth area is 4.46 × 3.46 = 15.43 m² while carpet area is 4.00 × 3.00 = 12.00 m².
Which is better, centre line or long wall–short wall method?
The centre line method is quicker when all walls have the same section. The long wall–short wall method is easier to check and suits buildings with walls of different thickness.