ZARONIA Calculator

Selecting ZARONIA presets the Market Practitioners Group's recommended loan conventions for South Africa's JIBAR successor: compounding in arrears on an Actual/365 basis with a 5 business day lookback and no observation shift, using the Johannesburg business day calendar. The advanced settings let you override any of these to match your facility agreement.

JIBAR is a forward-looking term rate: you know the rate at the start of the interest period. ZARONIA is an overnight rate compounded backward-looking in arrears, so the final rate is only known near the end of the period. That is why lookback conventions exist, and why legacy JIBAR contracts moving to ZARONIA typically add a credit adjustment spread on top of the margin. For where the market prices ZARONIA from here, see the ZARONIA forward curve.

Enter them separately: your margin in the margin field and the CAS in its own field, which appears whenever ZARONIA is selected. For facilities falling back from JIBAR, the fallback CAS was permanently fixed per tenor on 3 December 2025 under the Market Practitioners Group fallback methodology: 11.42bp for 1 month JIBAR, 16.19bp for 3 month, 43.23bp for 6 month, 58.30bp for 9 month and 74.10bp for 12 month. The field offers each in one click; bilaterally renegotiated deals should use whatever their agreement specifies. The calculator adds margin and CAS flat to compounded ZARONIA and returns the all-in rate and the interest amount on your notional. Note these presets follow the MPG loan convention with its 5 business day lookback; ISDA derivatives instead fall back to the Bloomberg Fallback Rate (ZARONIA), which shifts observations by 2 business days, so derivative fallback amounts differ slightly. You can approximate that convention in the advanced settings with a 2 business day lookback and observation shift enabled, but for settlement purposes the published Fallback Rate itself is the authoritative number.

Enter your interest period dates, the notional and your spread, and the calculator compounds the official SARB fixings in arrears. Worked example: a ZAR 50,000,000 drawing from 1 April to 30 June 2026 at ZARONIA plus 185bp, on an Actual/365 basis with a 5 business day lookback, gives compounded ZARONIA of 6.72763%, an all-in rate of 8.57763% and interest of ZAR 1,057,516.22. The period runs from 1 April inclusive to 30 June exclusive, 90 accrual days, with each day using the fixing observed 5 business days earlier per the MPG loan convention. The official SARB 3 month average for the same quarter reads about 2bp higher because it uses a 91 day window with no lookback. Enter the same inputs above and you should reproduce every figure.

The South African Reserve Bank publishes ZARONIA every Johannesburg business day, based on actual overnight deposit transactions. The calculator compounds those official published fixings; the daily series is on our ZARONIA fixings page, and daily compounded period averages are on our compounded ZARONIA page.