The formula
Drift band ≈ Amount × Daily move% × √Days ÷ 100
The square-root-of-time scaling reflects how uncorrelated daily moves accumulate: four days of transit produces roughly twice — not four times — the typical drift of one day, because moves partially cancel. It is the standard random-walk approximation used across market risk, applied here to a payment window.
The daily-move input is yours to estimate honestly: a pair’s recent average absolute daily change is a good anchor (major pairs often 0.3–0.6%, volatile emerging-market pairs 0.5–1.5%+). The calculator also shows the linear worst case — every day moving against you — which is unlikely but not impossible, and worth seeing for thin-margin payments.
How to use this calculator
- 1
Establish the transit window
Days between rate acceptance and settlement for your route — the Settlement Time Estimator decomposes it.
- 2
Estimate the pair’s daily move
Average absolute daily change over recent months; err high for emerging-market pairs and event weeks.
- 3
Read the band against your margin
A ±0.7% band on a 20%-margin payment is noise; on a 3%-margin distribution order it is a material fraction of profit.
- 4
Decide: lock, speed up, or accept
A committed quote removes the drift; a faster route shrinks it; accepting it is legitimate when the band is small relative to margin.
The hidden variable in provider comparisons
Provider comparisons usually stop at fees and quoted rates, silently assuming the quoted rate is the settled rate. Whether that is true depends on the provider’s model: quote-committed providers fix the rate at approval and carry the transit risk themselves; indicative-rate providers convert when funds arrive, leaving the drift with you. Two providers with identical quoted costs can differ by the full drift band in realised cost.
The comparison discipline: ask every provider whether the quoted rate is committed through settlement, and for how long the commitment holds. Then compare committed quotes against indicative quotes plus the drift band this calculator produces — that is the like-for-like comparison the fee table does not show.
Drift, deposits and long payment schedules
Supplier payment structures stretch drift over weeks: a 30% deposit converts today, the 70% balance converts at shipment six weeks later — the balance carries six weeks of drift, roughly 6.5 times the one-day band (√42 ≈ 6.5). For structured orders, drift risk concentrates in the later, larger legs, which is exactly where rate certainty is worth most.
The instruments match the legs: a committed quote covers each leg at execution; a forward or rate hold covers the balance leg from order date; and invoicing-currency negotiation can move the drift to the party better equipped to carry it. The Hedge vs No-Hedge Calculator prices the balance-leg decision; this calculator tells you how much risk is on the table.
Common use cases
Route economics
Add the drift band to a slow route’s cost before calling it cheaper than a fast one.
Provider model comparison
Compare committed-rate quotes against indicative rates plus drift, like for like.
Order leg planning
Size the drift on a deferred balance payment and decide whether to lock it.
Volatile-week awareness
Re-run with elevated daily moves around elections, rate decisions and data releases.
Automate this with the API
Anchor the drift band to a live indicative rate for your pair.
curl "https://keybs.io/api/v1/tools/fx?from=USD&to=NGN&amount=100000" \ -H "x-api-key: YOUR_FREE_KEY"Free Tools API docs and key registration
Frequently asked questions
Why square root of days rather than days?
Because daily moves partially cancel: a pair that moves 0.4% a day does not reliably move 2% in five days — up days offset down days. The √time scaling is the standard random-walk approximation for how volatility accumulates. The linear worst case is also shown as the bad-tail bracket.
Where do I get a daily-move figure for my pair?
Average the absolute daily percentage changes over the last three to six months from any rate history source. Round up in event-heavy periods. Major pairs typically run 0.3–0.6% daily; volatile emerging-market pairs 0.5–1.5% or more. The input is an estimate — test a higher figure to see sensitivity.
Is drift symmetric — could the rate move in my favour?
Yes, the band is two-sided: roughly equal chances of better and worse over short windows. The asymmetry is commercial, not statistical — an adverse move can erase a thin margin, while a favourable one merely pads it. Risk management prices the downside because the downside is what breaks budgets.
Does a committed quote really remove drift?
For the quoted window, yes — the provider commits the rate and carries the market risk through settlement. The residual questions are the window’s length and what happens if funding misses it. KeyBS Pay quotes state the rate, fee and receive amount with a validity window before approval; funding within the window fixes your outcome.
Should small payments worry about drift?
Usually not — a ±0.5% band on a $2,000 payment is ±$10, well under the attention threshold. Drift matters when amount × band approaches the payment’s fee cost or a meaningful slice of order margin. Run the calculator once per corridor to find your own threshold, then stop worrying below it.
How does stablecoin settlement change the drift picture?
It compresses the transit window — blockchain settlement in minutes leaves drift only on the on/off-ramp legs — but introduces its own conversion points whose pricing matters. The Stablecoin vs Bank Transfer Calculator compares the routes’ full cost; drift reduction is one of the stablecoin route’s quiet advantages on slow corridors.
Corridors, tools and reading for this calculator
More calculators
Replace assumptions with a committed quote
Executable rate, disclosed fee, committed receive amount — before you pay anything.
Request a Quote