Sri Lanka Treasury Bill (T-Bill) Yield Calculator

CBSL issues Treasury Bills at a discount to face value through a weekly primary auction. You bid a yield; if your bid clears, you pay the discounted purchase price upfront and receive the full face value at maturity. There are no coupon payments — the entire return is the discount captured.

The day-count convention is actual/365. From the annualised discount rate r and tenor t (in days), the purchase price PP is derived as:

PP = FV / (1 + r × t / 365)

Gross interest is simply FV − PP. From there, three different yield figures all describe the same economic return through different lenses:

• Money-market yield (365-day basis): (FV−PP) / PP × 365/t — this is the figure CBSL quotes as the auction weighted-average rate. By construction it equals the input r.
• Bank-discount yield (360-day basis): (FV−PP) / FV × 360/t — the US Treasury convention. Lower than the money-market yield because it divides by FV (not PP) and uses 360 days. Shown for reference only; you do not see this on CBSL.
• Effective annual yield (EAY): (FV/PP)^(365/t) − 1 — what you would earn over a full year if you kept rolling the bill at the same rate, with compounding. This is the only figure that compares apples-to-apples with a 1-year FD's AER.

Under the Inland Revenue Act No. 24 of 2017 (as amended), interest on T-bills paid to resident individuals is subject to a final 5% withholding tax, deducted at maturity by the primary dealer. The net interest received is Interest × 0.95 and the after-tax EAY is ((PP + NetInterest) / PP)^(365/t) − 1.

The calculator runs both directions — type a discount rate to see the implied purchase price, or type a purchase price to back out the implied yield. The independent cross-check (interest-on-cost formula) must equal the quoted auction rate to machine precision; a small “cross-check ✓” tag appears next to the EAY tile when the math reconciles.

Latest CBSL weighted-average auction rates cited on this page: 91-day 9.85%, 182-day 10.05%, 364-day 10.20% — snapshot dated 2026-05-16. Auctions clear every Wednesday; re-check the CBSL page before placing a bid.

• CBSL — Government Securities Market (auction mechanism & tenors)
• CBSL — Treasury Bill auction results & weighted-average rates
• CBSL — Public Debt Department (day-count and pricing convention)
• Inland Revenue Department — Withholding Tax on interest income

Formulas, day-count basis, and WHT treatment were last cross-checked against the CBSL and IRD primary sources on 2026-05-16. Auction rates are reviewed weekly; the cited weighted-average rates for 91 / 182 / 364-day tenors are a snapshot from the most recent published auction at the time of verification.