A case for using flow diagrams not just tables and ratios to trace how value, risk, and transactions actually move through an entity.
Most audit evidence is presented the same way it has been for decades: a table, a trial balance extract, a variance column, a percentage. These formats are precise, but they are also flat, they tell you what a number is, rarely where it came from or where it went next. A Sankey diagram does something different. It shows flow: how a quantity splits, merges, and narrows as it moves from a source to a destination, with the width of each band proportional to the amount moving through it. Named after the Irish engineer who first used the technique to chart steam engine energy losses, the format has quietly become one of the more useful and underused ways to present financial and operational movement in audit work.
A Brief History
The diagram takes its name from Captain Matthew Henry Phineas Riall Sankey, an Irish engineer and captain in the Royal Engineers, born in Nenagh, County Tipperary in 1853. In 1898, in a paper on the thermal efficiency of steam engines presented to the Institution of Civil Engineers, Sankey published a diagram that used arrows of varying width to show where the energy input to a steam engine was usefully converted to work and where it was lost — the widths drawn proportional to the actual quantities of energy involved. That single chart, showing how much heat was wasted at each stage of the engine's cycle, was enough to give the format its name.
Sankey was not, strictly, the first to draw this way. The French engineer Charles Joseph Minard had used proportional-width flow bands as early as the 1840s, most famously in his 1869 map of Napoleon's ill-fated 1812 Russian campaign arguably still one of the most admired statistical graphics ever produced predating Sankey's steam-engine chart by nearly three decades. But Minard's technique fell out of use after his death, while Sankey's version was picked up by German and Austrian engineers in the early twentieth century, particularly as post-war economies focused heavily on energy and material efficiency, and the name stuck to the method rather than to its earlier originator. From those industrial-efficiency roots, the format spread over the twentieth century into energy economics, environmental science, and more recently, with the rise of interactive data visualization tools into finance, process analytics, and audit.
Why Flow Beats Tables for Certain Audit Questions
A trial balance tells you the closing figure for trade receivables. It does not, on its own, tell you how much of that balance originated from related-party sales, how much aged into different buckets, or how much was subsequently written off versus collected. Answering that kind of question usually means building a pivot table and cross-referencing it manually. A Sankey diagram answers it visually in one view: the eye follows the band, not the formula. This matters most wherever an auditor is really asking "where did this go" rather than "how much is this" — cash flow tracing, fund utilization, revenue disaggregation, or a chain of intercompany transactions.
Practical Uses in an Audit Engagement
- Fund flow and utilization testing — tracing how proceeds from a debenture issue, rights issue, or bank facility were actually deployed across capex, working capital, and repayment of other debt, which is exactly the kind of test a debenture trustee or lender covenant review demands.
- Revenue disaggregation under Ind AS 115 — visualizing how a topline splits by product, geography, or timing of recognition (point-in-time vs over-time), making it easier to spot a segment where recognition policy doesn't match the disclosed pattern.
- Related-party transaction chains — mapping how a transaction moves through a promoter group's entities, which is often more revealing as a flow than as a list, because circular flows and round-tripping patterns are visually obvious in a Sankey but easy to miss in a schedule.
- Cost allocation and overhead tracing — showing how a shared service cost centre's expense splits across business units or projects, useful in cost audit and in testing whether project-wise revenue recognition is matched by a defensible cost allocation.
- Expected credit loss staging — following a loan book from origination through Stage 1, 2, and 3 classification into the resulting ECL provision, turning an otherwise abstract staging policy into something a reviewer can visually sanity-check.
- NPA and write-off progression — tracing how much of an opening NPA balance was upgraded, recovered, written off, or rolled forward, which is a standard LFAR-adjacent disclosure for bank audits that rarely gets a visual treatment.
Where It Adds the Most Audit Value
The real value of a Sankey diagram in an audit context is not aesthetic — it is diagnostic. A flow that should be a clean, narrowing funnel but instead shows an unexpected branch, a flow that loops back on itself, or a band that is disproportionately thick relative to its label, is often the fastest way to spot a control gap or a disclosure mismatch before a single substantive test is run. It functions the way analytical review is meant to function: a lens that directs attention to where the detail testing should concentrate, rather than a replacement for that testing.
A Few Practical Cautions
- A Sankey diagram is only as reliable as the underlying data mapping, it should be built from reconciled figures, not used as a substitute for the reconciliation itself.
- It works best for two to four levels of flow; beyond that, the diagram becomes as hard to read as the table it was meant to replace.
- It is a communication and risk-identification tool, not audit evidence in itself, the working paper still needs the underlying trace and sign-off.
The Broader Point
Audit methodology has always evolved alongside the tools available to test and present evidence from manual vouching to CAATs, from static schedules to now increasingly to data visualization. A Sankey diagram is a small but genuinely useful addition to that toolkit: not because it looks more sophisticated, but because certain audit questions are fundamentally about movement, and movement is easier to test once you can actually see it.
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