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.

Nothing is deducted at source. Section 84(3)(d) of the Inland Revenue Act No. 24 of 2017, consolidated to 31 March 2025, excludes interest or discount paid to any person on a Treasury Bill under the Local Treasury Bills Ordinance (Chapter 417), and Advance Income Tax under section 84A(1A) is expressly subject to that same subsection. The 0% shown in the calculator is not a concession, it is the absence of a charge. The 10% that applies to bank deposit interest from 1 April 2025 never reaches a T-bill, so you receive the full face value at maturity. The discount is still assessable income though, because section 88(1A) stopped treating interest paid to a resident individual as a final withholding payment in 2020, so declare it on your return and pay at your own progressive rate.

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 2026-10-17. Auctions clear every Wednesday; re-check the CBSL page before placing a bid.

Competitive vs non-competitive bids.At the weekly auction a Primary Dealer submits competitive bids that name the exact yield they want, or routes a smaller retail order as a non-competitive bid. Retail investors almost always go non-competitive: you accept whatever weighted-average rate the auction settles at rather than risk being shut out by bidding too aggressively. That weighted-average rate is the figure this calculator defaults to, and the one CBSL publishes each Wednesday. If you submit a competitive bid and it clears, switch the tool to “Enter price” mode and type the exact rupee price you paid to back out your own realised yield.

Selling before maturity. A T-bill is not locked away like a term deposit. It is a security held in your CDS account, so a Primary Dealer will buy it back on the secondary market any business day before maturity. The price then depends on prevailing yields: if market rates have risen since you bought, your bill changes hands at a lower price (a capital loss); if rates have fallen, it sells at a premium. This tool models buy-and-hold to maturity, where the rupee return is fixed the moment your bid clears. Secondary-market mark-to-market and premature-sale pricing are out of scope.

Where T-bills sit in your plan. Because the return is sovereign-guaranteed and the tenor is short, T-bills are a natural home for cash you cannot afford to lose but will not need for a few months. It is worth running the after-tax effective annual yield against a fixed deposit for the same period, the built-in comparison strip does exactly this, compounding an FD monthly per CBSL Direction No. 7 of 2011. The two are not taxed alike: an FD interest credit has 10% AIT taken at source, the rate the general withholding tax calculator applies to other interest income, while a T-bill is carved out of both WHT and AIT and pays out in full. Both remain assessable income at year end. If T-bill or FD interest is your only income for the year, you may still fall under the tax-free threshold, the Sri Lanka income tax calculator shows where your total assessable income lands.

• CBSL: Government Securities Market (auction mechanism & tenors)
• CBSL: Treasury Bill auction results & weighted-average rates
• CBSL: Public Debt Department (day-count and pricing convention)
• IRD: Inland Revenue Act No. 24 of 2017, consolidated to 31.03.2025 (s.84(3)(d) exclusion for Treasury Bills, s.84A(1A) AIT, s.88(1A), First Schedule para 10)
• Capital Alliance (CBSL-appointed primary dealer): tax comparison of Sri Lankan investments, T-bills carry no WHT against 10% on fixed deposits
• SimpleBooks: Sri Lanka withholding tax guide, reproduces the s.84(3)(d) government-securities exclusion

Formulas, day-count basis, and tax treatment were last cross-checked against the CBSL and IRD primary sources on 2026-10-17. 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.