Common calculation mistakes – and how to avoid them
Created December 2025, Reading time: approx. 2-3 minutes
Why calculation errors occur – and why they are so costly
Costing in construction projects is challenging because each project is unique. Even seemingly similar projects can have major differences in ground conditions, drawings, logistics, availability, progress, and material selection. Such differences have a direct impact on both costs and time spent.
In a busy everyday life, with short quotation deadlines and multiple parallel projects, the estimate often becomes a combination of professional knowledge, experience and assumptions – often based on an incomplete decision-making basis.
It is rarely the craftsmanship that causes financial problems. It is the calculation errors . Many of them seem small in isolation, but collectively they can eat up your entire profit. With low margins in the industry, accuracy, structure and control are essential for profitability.
1. Incorrect quantities – the most common and costly source of error
Quantity surveying is the foundation of any calculation . Errors here propagate directly to material costs, hours, logistics and rigging.
Common causes:
work based on incorrect or old drawings
details that are not registered (membrane, fittings, fasteners, accessories)
quantities based on assumptions rather than measurements
lack of shrinkage
lack of control over actual conditions on the construction site
hidden or insufficiently assessed conditions
How to avoid the error:
always use the latest revision of drawings and descriptions
compare drawings with inspection
establish fixed routines for quantity calculation per subject
document all assumptions (thicknesses, layer structure, shrinkage)
use checklists and digital calculation tools
2. Old or incorrect material prices – a significant risk factor
Material prices in the construction industry change rapidly. Lumber, steel, plaster, energy and transportation can fluctuate significantly in a short period of time.
Typical errors:
use of old Excel lists
verbal prices or "checklists"
outdated price lists
assumptions without documentation
Consequence:
Projects are priced too low, and the discrepancy is only discovered when the invoices arrive.
Measures:
use digital price banks or updated supplier prices
conduct price checks 4–6 times a year
be extra conservative with long construction times
documents what prices the offer is based on
3. Too low hours – idealized production meets reality
Underestimated hours are one of the most frequent causes of financial discrepancies.
Common mistakes:
logistics and availability are not included
waiting time for other subjects is missing
too little time for meetings, coordination and administration
finishing work, cleanup and completion are underestimated
Calculations are often based on theoretical production or standard times that do not take into account the actual conditions of the project.
How to reduce the risk:
use historical figures from your own projects
clearly distinguish between production time and support time
add buffer in case of high uncertainty
carry out post-calculations systematically
4. Indirect costs – the silent enemy of margins
Indirect costs are costs that cannot be tied to one specific item, but that accrue to the project over time. These are often the reason margins disappear without it being obvious why.
Examples of indirect costs:
car and fuel costs
tools, wear and service
insurance and fixed costs
construction power, heating and temporary installations
IT licenses and software
project management, HSE and administration
rigging and operation
How to deal with this:
establish a clear overhead model (mark-up, hourly or fixed project cost)
distinguish direct and indirect costs in the calculation
update the model annually based on actual figures
5. Forgotten project items – small amounts that destroy small margins
In many projects, concrete, project-specific costs are forgotten because they are perceived as small or "self-evident".
Typically forgotten items:
transportation and driving
container and waste management
small materials (glue, screws, tape, grout)
demolition and securing
scaffolding and temporary solutions
clearing and final cleaning
Solution:
use standardized checklists
create permanent records for rigging, waste and small materials
review the project phases: before, during and after
6. Lack of risk assessment – the calculation becomes too “pretty”
Projects that appear simple can contain significant uncertainty.
Typical risk areas:
Reconstruction and older buildings
weak or inadequate foundation
many subjects at the same time
short deadlines
weather-exposed works
customers with frequent changes
How to reduce the risk:
carry out risk assessments in all projects
add a risk premium (often 5–15%)
describe the reservations clearly in the offer
be conservative when uncertainty is high
Note: Risk surcharges are not the same as indirect cost surcharges. They cover different matters and must be considered separately.
7. Unclear offers – when good calculations yield poor results
A correct calculation loses value if the offer is unclear.
Common mistakes:
unclear scope
general descriptions
lack of reservations
unclear interfaces and responsibilities
Measures:
use fixed quote templates
clearly specify what is included and excluded
documents assumptions and basis
8. Lack of market control and bystander control
Even correct calculations can result in the wrong price in the market.
Recommendations:
compare your own prices against previous projects
perform sideman checks on larger offers
analyze both lost and won offers
9. Purchasing subcontractors (UE) – high cost, high risk
UE costs often constitute a significant portion of the project's total value.
Common mistakes:
too optimistic UE prices
unclear interfaces between disciplines
reservations that are not captured
lack of rigging and propulsion costs
How to reduce the risk:
obtain written offers with clear demarcations
compare more UE offers
check that propulsion, rigging and extras are included
10. No post-calculation – the industry's biggest missed improvement opportunity
Without post-calculation, the business is working blindly.
What the recalculation gives:
realistic experience figures
better hourly estimates
more precise quantities
better control over indirect costs
higher accuracy in new offers
Here's how to do it easily:
estimate all projects over a certain size
compare calculation against actual consumption
identify 3–5 main deviations
update routines and pricing and project templates
Checklist: Costs that are often forgotten in the calculation
Shrinkage
Real time spent vs. pure work operations
Preparation and preparation
Transport and deliveries
Tools and wear
Fasteners and small items
Waste and container
HSE / SHA
Rig and operation
General operating costs
Memory number:
As a rule of thumb, a minimum of 15% should be added to the calculated price for other project costs. A project of NOK 100,000 should therefore have approximately NOK 15,000 in additional costs, adjusted for the size, duration and complexity of the project.
The most common calculation errors are not random – they follow clear patterns. When companies introduce structure, checklists, better data bases and systematic routines, accuracy increases rapidly. The result is safer projects, fewer surprises and higher profitability.
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