By the end of this chapter, you will be able to:
These skills will help you manage construction costs effectively, ensuring your projects are both successful and profitable in the real trade.
Compiled by: John Wachira
Costing construction projects is a fundamental aspect of building technology that directly affects project feasibility and successful completion. Accurate cost estimation enables contractors, quantity surveyors, and project managers to plan budgets, allocate resources efficiently, and avoid financial losses. In the Kenyan construction environment, where market prices and material availability can fluctuate, using appropriate estimation methods is critical for competitive and realistic project proposals.
In Kenya’s building industry, selecting the right estimation method depends on project complexity, available data, and time constraints. Estimators commonly use the cubic method, unit cost method, and quantity cost method to align with project types ranging from small residential buildings to large commercial complexes. Understanding these methods ensures precise budgeting and cost control.
The cubic method estimates construction costs based on the building’s volume in cubic metres. This approach is suitable for preliminary costing when detailed drawings are unavailable but approximate dimensions are known. It assumes a standard cost per cubic metre, which varies with building type, location, and specification levels.
The formula for the cubic method is:
$$ \text{Estimated Cost} = \text{Volume of Building (m}^3) \times \text{Cost per m}^3 $$
Example 1: Estimate the cost of constructing a single-storey residential building with dimensions 10 m length, 8 m width, and 3 m height. Assume the cost per cubic metre is Ksh 15,000.
Given: Length = 10 m, Width = 8 m, Height = 3 m, Cost per m\(^3\) = Ksh 15,000
$$ \text{Volume} = 10 \times 8 \times 3 = 240 \text{ m}^3 $$
$$ \text{Estimated Cost} = 240 \times 15,000 $$
$$ = 3,600,000 \text{ Ksh} $$
Answer: Ksh 3,600,000
Example 2: A two-storey office building has a footprint of 20 m by 15 m and floor-to-floor height of 4 m. Calculate the approximate cost if the cost per cubic metre is Ksh 25,000.
Given: Length = 20 m, Width = 15 m, Height per floor = 4 m, Number of floors = 2, Cost per m\(^3\) = Ksh 25,000
$$ \text{Total Height} = 4 \times 2 = 8 \text{ m} $$
$$ \text{Volume} = 20 \times 15 \times 8 = 2,400 \text{ m}^3 $$
$$ \text{Estimated Cost} = 2,400 \times 25,000 $$
$$ = 60,000,000 \text{ Ksh} $$
Answer: Ksh 60,000,000
Example 3: Estimate the cost of a warehouse with dimensions 30 m length, 25 m width, and 6 m height. The cost per cubic metre is Ksh 10,000.
Given: Length = 30 m, Width = 25 m, Height = 6 m, Cost per m\(^3\) = Ksh 10,000
$$ \text{Volume} = 30 \times 25 \times 6 = 4,500 \text{ m}^3 $$
$$ \text{Estimated Cost} = 4,500 \times 10,000 $$
$$ = 45,000,000 \text{ Ksh} $$
Answer: Ksh 45,000,000
Example 4: A residential block has a volume of 1,200 m\(^3\) and the estimated cost per cubic metre is Ksh 18,500. Calculate the estimated cost.
Given: Volume = 1,200 m\(^3\), Cost per m\(^3\) = Ksh 18,500
$$ \text{Estimated Cost} = 1,200 \times 18,500 $$
$$ = 22,200,000 \text{ Ksh} $$
Answer: Ksh 22,200,000
Example 5: Calculate the estimated cost for a building with volume 3,750 m\(^3\) and cost per cubic metre Ksh 22,000.
Given: Volume = 3,750 m\(^3\), Cost per m\(^3\) = Ksh 22,000
$$ \text{Estimated Cost} = 3,750 \times 22,000 $$
$$ = 82,500,000 \text{ Ksh} $$
Answer: Ksh 82,500,000
The unit cost method estimates the total cost by multiplying the quantities of various building components by their respective unit costs. This method is detailed and suited for projects with available design drawings and specifications. It allows more accuracy than the cubic method by breaking down the building into measurable elements.
The general formula is:
$$ \text{Estimated Cost} = \sum (\text{Quantity of Element} \times \text{Unit Cost of Element}) $$
Example 1: Calculate the cost of flooring for a building with 150 m\(^2\) floor area. The unit cost of floor finishing is Ksh 2,500 per m\(^2\).
Given: Floor area = 150 m\(^2\), Unit cost = Ksh 2,500/m\(^2\)
$$ \text{Cost} = 150 \times 2,500 $$
$$ = 375,000 \text{ Ksh} $$
Answer: Ksh 375,000
Example 2: Estimate the cost of wall construction for 200 m\(^2\) area with unit cost Ksh 3,800 per m\(^2\).
Given: Wall area = 200 m\(^2\), Unit cost = Ksh 3,800/m\(^2\)
$$ \text{Cost} = 200 \times 3,800 $$
$$ = 760,000 \text{ Ksh} $$
Answer: Ksh 760,000
Example 3: For a building, the quantities and unit costs are:
Calculate total estimated cost.
Given:
Roof area = 120 m\(^2\), Unit cost = Ksh 4,500/m\(^2\)
Wall area = 180 m\(^2\), Unit cost = Ksh 3,800/m\(^2\)
Floor area = 120 m\(^2\), Unit cost = Ksh 2,500/m\(^2\)
$$ \text{Roof cost} = 120 \times 4,500 = 540,000 \text{ Ksh} $$
$$ \text{Wall cost} = 180 \times 3,800 = 684,000 \text{ Ksh} $$
$$ \text{Floor cost} = 120 \times 2,500 = 300,000 \text{ Ksh} $$
$$ \text{Total cost} = 540,000 + 684,000 + 300,000 = 1,524,000 \text{ Ksh} $$
Answer: Ksh 1,524,000
Example 4: A contractor is estimating costs for plastering 250 m\(^2\) of walls at Ksh 600 per m\(^2\), painting 250 m\(^2\) at Ksh 350 per m\(^2\), and tiling 100 m\(^2\) at Ksh 1,200 per m\(^2\). Find total cost.
Given:
Plastering area = 250 m\(^2\), Unit cost = Ksh 600/m\(^2\)
Painting area = 250 m\(^2\), Unit cost = Ksh 350/m\(^2\)
Tiling area = 100 m\(^2\), Unit cost = Ksh 1,200/m\(^2\)
$$ \text{Plastering cost} = 250 \times 600 = 150,000 \text{ Ksh} $$
$$ \text{Painting cost} = 250 \times 350 = 87,500 \text{ Ksh} $$
$$ \text{Tiling cost} = 100 \times 1,200 = 120,000 \text{ Ksh} $$
$$ \text{Total cost} = 150,000 + 87,500 + 120,000 = 357,500 \text{ Ksh} $$
Answer: Ksh 357,500
Example 5: Calculate the estimated cost for a building with the following elements:
Given:
Doors = 10 units, Unit cost = Ksh 15,000
Windows = 20 units, Unit cost = Ksh 8,000
Electrical outlets = 50 units, Unit cost = Ksh 3,500
$$ \text{Doors cost} = 10 \times 15,000 = 150,000 \text{ Ksh} $$
$$ \text{Windows cost} = 20 \times 8,000 = 160,000 \text{ Ksh} $$
$$ \text{Electrical outlets cost} = 50 \times 3,500 = 175,000 \text{ Ksh} $$
$$ \text{Total cost} = 150,000 + 160,000 + 175,000 = 485,000 \text{ Ksh} $$
Answer: Ksh 485,000
The quantity cost method involves detailed measurement of quantities of materials, labour, and plant, multiplying each by respective rates to derive the total project cost. This method provides the most accurate estimate and is suitable for tendering and cost control. It requires comprehensive bills of quantities and current market rates.
The formula is:
$$ \text{Total Cost} = \sum (\text{Quantity} \times \text{Rate}) $$
where quantity and rate cover materials, labour, and equipment.
Example 1: Calculate the cost of 500 bags of cement priced at Ksh 700 each.
Given: Quantity = 500 bags, Rate = Ksh 700/bag
$$ \text{Cost} = 500 \times 700 $$
$$ = 350,000 \text{ Ksh} $$
Answer: Ksh 350,000
Example 2: Labour cost for plastering 300 m\(^2\) walls at Ksh 250 per m\(^2\).
Given: Quantity = 300 m\(^2\), Rate = Ksh 250/m\(^2\)
$$ \text{Cost} = 300 \times 250 $$
$$ = 75,000 \text{ Ksh} $$
Answer: Ksh 75,000
Example 3: Estimate the cost of 2,000 bricks at Ksh 80 each and labour for laying bricks at Ksh 150 per 100 bricks.
Given: Bricks quantity = 2,000, Rate = Ksh 80/brick
Labour rate = Ksh 150 per 100 bricks
$$ \text{Materials cost} = 2,000 \times 80 = 160,000 \text{ Ksh} $$
$$ \text{Labour cost} = \frac{2,000}{100} \times 150 = 20 \times 150 = 3,000 \text{ Ksh} $$
$$ \text{Total cost} = 160,000 + 3,000 = 163,000 \text{ Ksh} $$
Answer: Ksh 163,000
Example 4: Calculate total cost for:
Given:
Cement: 100 bags, Ksh 700/bag
Labour: 5 workers, 10 days, Ksh 1,200/day
Mixers: 2 units, 5 days, Ksh 3,000/day
$$ \text{Cement cost} = 100 \times 700 = 70,000 \text{ Ksh} $$
$$ \text{Labour cost} = 5 \times 10 \times 1,200 = 60,000 \text{ Ksh} $$
$$ \text{Mixer cost} = 2 \times 5 \times 3,000 = 30,000 \text{ Ksh} $$
$$ \text{Total cost} = 70,000 + 60,000 + 30,000 = 160,000 \text{ Ksh} $$
Answer: Ksh 160,000
Example 5: Calculate the total cost of the following:
Given:
Sand: 15 m\(^3\), Ksh 3,000/m\(^3\)
Gravel: 10 m\(^3\), Ksh 4,500/m\(^3\)
Labour: 8 workers, 12 days, Ksh 1,000/day
$$ \text{Sand cost} = 15 \times 3,000 = 45,000 \text{ Ksh} $$
$$ \text{Gravel cost} = 10 \times 4,500 = 45,000 \text{ Ksh} $$
$$ \text{Labour cost} = 8 \times 12 \times 1,000 = 96,000 \text{ Ksh} $$
$$ \text{Total cost} = 45,000 + 45,000 + 96,000 = 186,000 \text{ Ksh} $$
Answer: Ksh 186,000
Create a free account to open more of this chapter.
Free: practical guides, quick cards, workplace scenarios and more.
Create a free accountThis chapter covered the essential methods of estimation used in construction projects, including the cubic method, unit cost method, and quantity cost method, each offering different approaches to calculating project costs. The preparation of specifications was detailed, emphasizing the importance of clearly defining material requirements and workmanship standards to ensure quality and consistency. The process of preparing a rate schedule was explained as a critical step in organizing and standardizing unit costs for various construction activities. Finally, the chapter addressed the compilation of the bill of quantities, which serves as a comprehensive document listing all materials, labor, and costs required for the project, facilitating accurate tendering and project management. Understanding these components is fundamental for effective cost control and successful project delivery in building works.
A rectangular foundation has dimensions 5 m by 3 m and a depth of 0.5 m. Using the cubic method, calculate the volume of concrete required in cubic metres. (2 marks)
A wall requires 50 square metres of plastering. If the unit cost of plastering per square metre is Ksh 450, estimate the total cost. (2 marks)
At the start of this chapter we promised you would be able to:
Tick each one you can genuinely do.
So, are you there yet?
You're competent when you can confidently do 50% or more of what this chapter promised.
Sign in to record how you're doing.