How we calculate
Every number on this site comes from a published source and a formula you can check yourself. This page lists both.
Where the numbers come from
| T-bill rates | Central Bank of Kenya weekly auction results. Latest: issue dated 12 Oct 2026. |
| Central Bank Rate and inflation | Central Bank of Kenya, which publishes the inflation figure from KNBS. Latest inflation: 6.8% (Sep 2026). |
| Exchange rates | CBK indicative mean rates. Canadian and Australian dollar rates are worked out from the CBK US dollar rate and European Central Bank reference rates. Latest: 9 Oct 2026. |
| Money market fund yields | Published by each fund manager, collected every week. Latest: 13 October 2026. |
| Money market fund sizes | CMA Collective Investment Schemes Report, Q2 2026 (as at 30 June 2026). |
| Salary deductions | Income Tax Act and Finance Acts (KRA), the Social Health Insurance Act (SHA), the NSSF Act and the Affordable Housing Act. Rules as at October 2026. |
| Rental income tax | KRA guidance on Monthly Rental Income tax. |
When they update
- T-bill rates, Central Bank Rate, inflation and exchange rates: checked automatically every Friday and Saturday morning, after the Thursday auction.
- Money market fund yields: every week.
- Fund sizes: every quarter, when the Capital Markets Authority publishes its report.
- Tax and payroll rules: when a new law or KRA notice takes effect.
Every page shows the date of the figures it uses. Calculators and examples are built from the same data, so they always agree with each other.
The formulas
Treasury bills
- Price = face value ÷ (1 + rate × days ÷ 365). This is the CBK's 365-day discount method.
- Interest = face value − price. Withholding tax = 15% of the interest, paid upfront.
- Net annual return = (interest − tax) ÷ (price + tax) × 365 ÷ days.
Example: at 9.0352%, a 364-day bill returns 7.58% a year after tax.
Treasury and infrastructure bonds
- Each coupon = face value × coupon rate ÷ 2, paid every six months.
- Withholding tax on coupons: 15% for bonds under 10 years, 10% for 10 years or more, and none for infrastructure bonds.
Money market funds
- Net yield = quoted yield × 0.85 (after 15% withholding tax). Quoted yields are annual and after the fund's fees.
- Value after a period = amount × (1 + net yield) ^ (months ÷ 12). Real funds credit interest daily or monthly, so actual results differ slightly.
Real return
Real return = (1 + return) ÷ (1 + inflation) − 1. It shows what your money earns after rising prices.
Salary
- NSSF: 6% of pay up to KSh 108,000 (Tier I on the first KSh 9,000).
- SHIF: 2.75% of gross pay, minimum KSh 300.
- Housing Levy: 1.5% of gross pay.
- Taxable pay = gross pay − NSSF − SHIF − Housing Levy.
- PAYE = tax on taxable pay using the monthly bands, less personal relief of KSh 2,400.
We don't include pension contributions, insurance relief or mortgage interest relief, because they differ for each person.
Loans
- Reducing balance: payment = amount × i ÷ (1 − (1 + i) ^ −months), where i = annual rate ÷ 12. Interest is charged only on what you still owe.
- Flat rate: interest = amount × annual rate × years, charged on the original amount for the whole loan.
Rental income tax
Monthly Rental Income tax = 7.5% of gross rent received, for residential rent between KSh 288,000 and KSh 15 million a year. It's due by the 20th of the following month.
Rent vs buy and investing from abroad
These compare two paths month by month using the growth, return and exchange-rate assumptions you enter. They show the effect of your assumptions, not a forecast.
Rounding and limits
- Shilling amounts are rounded to the nearest shilling, or to the cent in T-bill examples. Percentages are rounded to two decimal places.
- Your bank, fund or employer may round differently or charge fees we don't know about.
- Calculators are tested against worked examples before they go live.
Corrections
If we find an error, we fix it and record it here.
| 15 October 2026 | The T-bill worked examples on the T-bill calculator and the DhowCSD guide still showed the 28 September auction rate after newer rates were published. They now update automatically with each auction. |
Spotted something wrong or out of date? Tell us or message us on WhatsApp.
For education only, not financial or tax advice. Rules and rates change: always check with CBK, KRA or your provider before you act.