How-To Guides · For yards, quarries & crews

Stockpile Reconciliation: Why Year-End Write-Offs Happen and How to Make Your Number Defensible

October 24, 2026 · 7 min read

Every December some operation discovers that the pile is smaller than the books. Here is where the gap comes from, what the auditors will ask, and how to build a stockpile number you can defend.

In short

Stockpile reconciliation compares the book quantity (last count plus production or deliveries minus sales) with a measured quantity. The difference is the variance, and at year end an unexplained variance becomes a write-off.
Seven things drive the gap: survey error, wrong density, the base surface, timing, rehandle and haul-back, unit confusion between tonnes and cubic metres, and recording errors or theft.
Auditors attend the count, judge the surveyor, read the measurement report, trace sample piles to the listing and test the density factor. A number without that paper trail is a guess with a signature.
Frequent cheap measurements plus one careful annual survey beat a single year-end survey. The monthly numbers find the cause while it still exists.

What reconciliation is

Stockpile reconciliation compares two numbers that should agree and rarely do. The book quantity is what the pile should contain, worked forward from the last measurement by adding production or deliveries and subtracting sales, all from tickets. The measured quantity is what a survey, drone flight or scan says is on the ground, converted to tons through a density. The difference is the variance. Through the year it is a management number; at the financial year end a variance nobody can explain is written off against profit.

Propeller’s review of year-end stockpile reconciliations calls large December write-offs “par for the course” where the monthly numbers were guesses: a flyover every six months and haul data in between. The write-off is not the loss; it is the moment the loss is finally measured.

The seven causes of a stockpile write-off

Guides from Birdi and Propeller’s reconciliation webinar list the same culprits.

Survey error. A tape-and-cone estimate can be 15–30% out; a sparse ground survey misses hollows; a scan is only as good as its coverage of the toe.
Density. Volume becomes tons through a factor that was chosen once and never checked. A 7% density error is a 7% inventory error on every pile.
The base surface. LiDAR Magazine’s field experience is that when a pile disagrees with production records, the definition of the base is usually the primary culprit: a survey that assumes last year’s pad under a pile sitting on this year’s leftovers invents or deletes material.
Timing. The survey was Tuesday, the books closed Friday, 900 tons shipped in between.
Rehandle and haul-back. Material moved between piles, blended, or taken to a job and returned, without a ticket each way.
Units. Tonnes recorded as tons, cubic metres as cubic yards, loose yards as bank yards. A 10% unit mismatch is invisible until the count.
Recording errors and theft. Missed tickets, mistyped quantities, material out the gate without paperwork.

The first three are measurement problems, fixed by measuring better. The rest are process problems, found only by measuring more often: each leaves a signature in the month it happens that is gone by December.

What the auditors actually do

Auditors do not take the stockpile number on trust. The clearest public description of what they do instead is a listed aggregates company’s annual report, where stockpile quantities were a key audit matter. In the auditor’s report, management measured year-end stockpiles with external surveyors and converted volumes to quantities with a density factor. The auditors attended the physical count, assessed the competence and objectivity of the surveyors, read their measurement reports, traced sampled piles from the count to the final inventory listing, considered whether the density factors were reasonable, and recomputed the conversion arithmetic.

For a smaller operation the checklist is the same. Who measured, with what, and could they be relied on. Is there a dated report per pile with a method and photos. Does the listing agree with the reports. Where did the density come from, and does it match the scale. Can the arithmetic be re-run. Answer those five and the number is defensible; a spreadsheet of volumes typed from memory is not, however good the volumes were.

Monthly versus annual

Propeller recommends auditing aggregate inventory monthly and calls quarterly too infrequent; its illustration is a $6–7 per ton gap across 10,000 tons, a $60–70k problem. The deeper argument is diagnostic. A variance found in the month it occurred has a short list of suspects: the wet fortnight, the new loader operator, the blended pile. Found at year end, it has twelve months of suspects and no witnesses. Monthly measurement need not be survey-grade; it needs to be consistent, dated and cheap enough to happen. The annual survey then confirms the big number rather than discovering it.

A worked example in dollars

A yard carries a book quantity of 10,000 tons of ¾-inch crushed stone at $14 per ton. The year-end scan measures 6,350 cubic yards. At the book density of 1.5 tons per cubic yard that is 9,525 tons, a 475-ton variance and a $6,650 write-off. But the scale says a level bucket of this stone weighs 1.40 tons per cubic yard, making the pile 8,890 tons, a 1,110-ton variance and a $15,540 write-off. The density assumption alone moved the write-off by $8,900, more than the measurement did. Add a base defined as last year’s pad, with 400 cubic yards of old fines counted as new stone, and that is another $8,400. The scan was fine; the number around it was not.

Cause	How to detect it	How to prevent it
Survey error	Re-measure a sample pile by a second method; compare ± ranges	Full-coverage scans; walk the whole toe; measure still piles
Wrong density	Weigh a known volume on the scale and compare with the factor	Measure density per product and season; record its source
Base surface	Compare footprints and pad elevation across surveys	Fix the base rule in writing and use it every time
Timing	Variance tracks shipments between survey and close	Measure on the closing date, or roll forward with that day’s tickets
Rehandle and haul-back	One pile long, a neighbor short	Ticket every internal move; name piles consistently
Unit confusion	Variances near 10% (tonne/ton) or 31% (m³/yd³)	One unit system in the ledger; convert at entry
Recording error or theft	Sudden drop with no tickets	Gate procedure; cameras; monthly count narrows the window

Detection is easiest in the month the variance appears, which is the argument for frequent measurement.

Building a defensible number

Defensible means reproducible. Each element below closes a question an auditor or a buyer would otherwise ask.

A written base rule, the same for every pile and every month: the pad surface on the day of the count, or a surveyed floor, but not sometimes one and sometimes the other.
Dated scans with photos for every pile, so the count can be re-examined later.
Saved borders. The outline used to define the pile is part of the measurement; keep it with the volume.
A documented density source per product, with the date and method of its measurement.
A variance log of each month’s difference and its explanation, so the year-end number has a history rather than a surprise.
Ranges, not points. 6,350 ± 190 cubic yards tells the reader how much to trust it; a bare 6,350 invites an argument about the last digit.

The guide to stockpile measurement accuracy explains where that ± range comes from and which factors widen it.

Where phone scans fit

Most operations end up layered. An annual or semi-annual drone or ground survey, often by an outside firm, produces the big number the auditors see; our drone versus phone comparison covers when that is worth it. In between, phone scans keep the monthly variance log alive at almost no cost, reach piles under roofs and in bins, and settle delivery-day disputes. Stockpile Scan produces a dated PDF per pile with the volume, its ± range, the border, photos and the density used, most of the paper trail above as a by-product of measuring. See the monthly count workflow for small yards and the overview of measurement methods.

Frequently asked questions

What is stockpile reconciliation?

Stockpile reconciliation compares the book quantity of a stockpile, worked forward from the last count using production, delivery and sales tickets, with a measured quantity from a survey, drone flight or scan converted to tons through a density. The difference is the variance. At the financial year end, an unexplained variance is written off.

Why do stockpile inventory write-offs happen?

Seven causes account for most of them: measurement error, a wrong density factor, an inconsistent base surface under the pile, timing gaps between the survey and the books, rehandled or hauled-back material without tickets, unit confusion between tonnes and tons or cubic metres and cubic yards, and recording errors or theft. Infrequent measurement lets them accumulate unseen until year end.

How often should stockpiles be measured for inventory?

Monthly for management purposes, with a more rigorous annual survey for the financial statements. Quarterly is generally considered too infrequent, because a variance found three months late has too many possible causes to diagnose. Frequent, consistent, cheap measurements find the cause while it still exists.

What do auditors need for a stockpile count?

In a published audit of an aggregates company, the auditors attended the year-end count, assessed the competence and objectivity of the surveyors, read the measurement reports, traced sampled piles to the inventory listing, assessed whether the density factors were reasonable and recomputed the conversion. A per-pile report with date, method, photos, border and density source, plus a documented density measurement, answers those tests.

Measure your next pile with your iPhone

Stockpile Scan is free, with no ads, subscriptions or account. Each scan produces a dated PDF with the volume, its ± range, the border, photos and the density used: a paper trail as a by-product of measuring.

Download on the App Store

Any iPhone with iOS 18 or later. LiDAR optional. Learn more about the app.