If you have ever been quoted a Treasury Bill rate and wondered exactly what it means for your money, this article gives you the same formula the Bank of Ghana itself uses, so you never have to take a quote on faith.
The Formula
Purchase Price = Face Value ÷ (1 + (Rate × Days ÷ 365))
Once you know the purchase price, your return is simply:
Return = Face Value − Purchase Price
Four things go into the calculation: the face value (what you receive at maturity), the rate set at that week's auction, the number of days in your chosen tenor (91, 182, or 364), and 365, used to scale the rate to the actual length of your tenor. That is the entire calculation.
A Full Worked Example
At the Bank of Ghana's auction on Friday, 5 July 2026, the 91-day rate was set at 24.78%. Using that real, dated rate, a GH₵10,000 face value 91-day Treasury Bill would have cost:
Rate × Days ÷ 365 = 0.2478 × 91 ÷ 365 = 0.0618 1 + 0.0618 = 1.0618 Purchase Price = 10,000 ÷ 1.0618 = GH₵9,418 Return = 10,000 − 9,418 = GH₵582
So, on this specific date, at this specific rate, an investor paying GH₵9,418 would have received GH₵10,000 back 91 days later, a return of GH₵582. This rate is a real, dated teaching example, not a current figure. Rates change every week; check the live rate at bog.gov.gh/treasury-and-the-markets before investing, and redo this same calculation with today's number. A dedicated Treasury Bill calculator does not currently exist on Robayer WealthLab's calculators page, which would make this exercise even faster; until then, this formula and any basic calculator, including the one on your phone, will do the job.
How Tenor Changes the Return
The same formula, applied to the 182-day and 364-day rates from that same 5 July 2026 auction, on the same GH₵10,000 face value:
| Tenor | Rate (5 July 2026) | You would pay | You would receive | Return |
|---|---|---|---|---|
| 91-day | 24.78% | GH₵9,418 | GH₵10,000 | GH₵582 |
| 182-day | 26.74% | GH₵8,824 | GH₵10,000 | GH₵1,176 |
| 364-day | 27.78% | GH₵7,831 | GH₵10,000 | GH₵2,169 |
Notice the longer tenor does not simply double or triple the shorter tenor's return; it grows because a higher rate applies and because the money is committed for longer. See our full Treasury Bill Tenors comparison for how to choose between them.
What You Actually Keep: Tax
Interest earned on Government of Ghana Treasury Bills is currently exempt from income tax for individuals, under the Income Tax Act, 2015 (Act 896), Section 7(p) and (r), as amended. In plain terms, the return in the table above is not reduced by any further deduction; what you calculate is what you keep. This exemption is current policy, not a permanent guarantee; there has been public discussion in Ghana about whether it should eventually be removed to raise tax revenue, so it is worth confirming the position remains unchanged before making a major decision based on it.
The One Mistake to Avoid
Do not compare a Treasury Bill's quoted rate directly to a savings account's advertised annual interest rate as though they work the same way. A savings account typically adds interest onto your balance periodically. A T-bill's rate feeds into the discount formula above, applied over the specific number of days in your chosen tenor, not automatically over a full year. Always run the actual calculation for your actual tenor rather than assuming the quoted percentage is simply "what you'll earn in a year."
FAQ
Do I need to do this math myself to invest? No. Your bank or broker will always confirm the exact price before you commit. This formula exists so you can verify their number, not as a requirement to invest.
Is the return the same no matter which bank I use? The formula and the auction rate are the same for everyone; any difference would come from a handling fee, which varies by institution and is worth asking about directly.
Does the tax exemption apply to every investor? Currently, yes, for individuals and non-resident persons, under the cited section of the Income Tax Act. Confirm this remains current before relying on it.