Bill of Quantities Errors: 3 Silent Mistakes That Cost Money
The costliest bill of quantities errors are not arithmetic mistakes but silent ones: a dimension that was assumed instead of read, an element counted twice across sheets, or an element that never reached the takeoff at all. These errors produce numbers that look complete and plausible, so they survive normal review and surface only during construction, when the concrete or steel runs short. Each one can be detected before submission with a deliberate check.
What are the most costly errors in a bill of quantities?
The most costly errors in a bill of quantities are assumed dimensions, double counting and missing elements, because all three change quantities without leaving any visible trace. An arithmetic slip usually produces an odd-looking number that someone notices; a silent error produces a normal-looking one.
The table below summarises the three, using illustrative round numbers only.
| Silent error | Typical cause | Illustrative effect | How to catch it |
|---|---|---|---|
| Assumed dimension | Thickness or spacing stated only in a section, note or legend | 100 m² slab taken at 200 mm instead of 250 mm: 5 m³ of concrete missing | List every value that was not read directly from a drawing |
| Double counting | Same element shown on a plan, a section and a detail | One 10 m beam's rebar billed twice: about 173 kg extra | Assign each element to a single source sheet |
| Missing element | Items shown only on foundation sheets, schedules or typical details | 20 piles Ø80 cm × 15 m: about 151 m³ of concrete missing | Reconcile the takeoff with schedules and the foundation plan |
How do assumed dimensions distort a bill of quantities?
Assumed dimensions distort a bill of quantities by replacing a value the drawing defines somewhere else with a value the estimator or software expects. The assumption is usually reasonable, which is precisely why nobody questions it.
Illustrative example: a 100 m² slab whose thickness appears only in a section. If the takeoff assumes a common 200 mm, the slab line shows 100 × 0.20 = 20 m³. If the section actually shows 250 mm, the true volume is 100 × 0.25 = 25 m³. The bill is 5 m³ short, a fifth of the real volume, and the same gap repeats on every floor that uses that slab type.
Rebar spacing behaves the same way. Illustrative example: one layer of Ø12 bars in one direction across a 10 m wide slab, with bars 10 m long. At an assumed 200 mm spacing, the layer has 51 bars, or 510 m and about 453 kg. At the 150 mm spacing actually specified in a note, it has 68 bars, or 680 m and about 604 kg. One unread note removes about 151 kg from a single layer.
Typical places where values get assumed:
- Slab and raft thicknesses defined in sections or general notes.
- Bar spacing and diameter given as "typical unless noted otherwise".
- Footing depths shown only by levels, not by a written dimension.
- Steel profiles listed in a legend rather than on each member.
The method for finding thicknesses in drawings is covered in concrete quantity calculation, and bar spacing and length rules in rebar quantity calculation.
How does the same element get counted twice?
The same element gets counted twice when it appears on more than one sheet and each appearance is measured as a separate element. Construction drawings deliberately repeat information, so double counting is a built-in risk of every complete set.
Common situations include:
- Plan and detail: a beam is drawn on the floor plan and again on a beam detail sheet, and both are measured.
- Overlapping sheets: large floors split across several sheets share an overlap zone near the match lines.
- Revisions: an old and a new revision of the same sheet are both included in the set.
- Shared zones: the part of a beam inside a slab is counted in the beam and in the slab.
Illustrative example: a 10 m beam with about 173 kg of rebar, measured once from the plan and once from its detail, appears in the bill as about 347 kg. On a single beam the difference is modest; across a typical floor repeated over many storeys, an overpriced bid can lose a tender just as surely as an underpriced one can lose money.
Which elements fall out of a bill of quantities entirely?
The elements that most often fall out of a bill of quantities entirely are those drawn on separate sheets or in schedules rather than on the main plans: piles, pile cages, starter bars, pedestals and anything defined in a typical detail. Because nothing is measured, nothing looks wrong.
Illustrative example: a foundation with 20 piles of Ø80 cm, each 15 m long. Each metre of Ø80 cm pile holds 0.503 m³, so each pile holds 0.503 × 15 = 7.545 m³ and the 20 piles together about 151 m³. If each pile cage has 10 longitudinal Ø20 bars running the full 15 m, the cages also hold 20 × 10 × 15 × 2.466 = 7,398 kg, about 7.4 t of rebar, before any spiral or links are counted. If the pile schedule sits on a sheet that never reached the takeoff, all of this is simply absent.
Typical details are the other major source. A detail for a typical slab edge or column starter is drawn once but applies at dozens of locations, and its quantities belong to every one of them.
How do you detect these errors before submitting a bill of quantities?
These errors are detected before submission by checking each quantity against its source, not just its arithmetic. A focused review in the following order catches most of them:
- List the assumptions. Write down every value in the takeoff that was not read directly from a drawing, and confirm each one with the source sheet or the designer.
- Map elements to sheets. Assign every element to one source sheet, and look for elements that were measured from two.
- Check the revision list. Make sure only the latest revision of each sheet is in the set.
- Reconcile with schedules. Compare pile, column, beam and bar schedules with the takeoff, element by element.
- Apply typical details. For each typical detail, count the locations where it applies and confirm the quantities were multiplied accordingly.
- Run sense checks. Compare ratios such as rebar per cubic metre of concrete, or concrete per square metre of floor, against what is typical for the structure.
The full ten-point version of this review is in the bill of quantities checklist.
Why is asking better than assuming?
Asking is better than assuming because a question makes the uncertainty visible to the people who can resolve it, while an assumption hides it inside a number. A question costs a few minutes; a wrong assumption is discovered on site, when it costs a variation, a delay or a margin.
This is the principle behind Quantap: Quantap doesn't guess, it asks. When something in the drawing is unclear, missing or contradictory, such as a thickness that appears nowhere or two sheets that disagree, Quantap asks the user through Quick Check, and the quantities update with the answers. Every line in the report can be checked back against the drawing, which makes double counting and missing elements easier to spot. Uploading the complete set, with plans, sections, elevations and typical details, gives it the sources it needs. The wider workflow is described in the guide to the automated bill of quantities, and common questions are answered in the FAQ.
Summary
The bill of quantities errors that cost the most are assumed dimensions, double counting and missing elements, and none of them shows up as an obviously wrong number. Assumed values hide in sections, notes and legends; duplicates come from information repeated across sheets; and missing elements live on foundation sheets, schedules and typical details. A review that traces each quantity to its source and treats every unresolved value as an open question catches them before submission. The discipline is simple: if a value was not read from the drawing, it has to be confirmed.
Frequently asked questions
What is the difference between a calculation error and a hidden assumption?
A calculation error is wrong arithmetic on correct inputs, and it is usually caught by a second check. A hidden assumption is correct arithmetic on an input nobody read from the drawing, so the result looks right and survives a normal review.
Who is responsible for errors in a bill of quantities?
Responsibility for errors in a bill of quantities depends on the contract and on who prepared the bill. In every case, the estimator who submits the quantities should be able to show where each one came from.
Can automated takeoff software introduce these errors too?
Automated takeoff software can introduce the same silent errors if it fills missing values with defaults or reads the same element from several sheets. Software that flags unclear items and links each line to the drawing makes them far easier to catch.
