Fidamen

Treasury Bill (T-Bill) Calculator

This calculator supports the three common T‑Bill workflows: start from the quoted bank discount rate, from an observed market purchase price, or from a bond‑equivalent (investment) yield. Results include the purchase price, profit at maturity, the bank discount yield (quoted on a 360‑day basis) and the bond‑equivalent yield (annualized on actual/365).

Defaults follow market conventions used for U.S. Treasury bills: bank discount quotes use a 360‑day base; bond‑equivalent yields are annualized on 365 days. Use the method selector to switch workflows and enter values as decimals for rates (for example, 0.05 = 5%).

Updated Nov 22, 2025QA PASS — golden 25 / edge 120Run golden-edge-2026-01-23

Governance

Record 3517c70d8f49 • Reviewed by Fidamen Standards Committee

Compute purchase price and equivalent yields given the bank-discount convention (d) used for T‑bills.

Inputs

Advanced inputs

Inputs for discount-rate mode

Inputs for price mode

Inputs for yield mode

Results

Updates as you type

Purchase price

$987.36

Profit at maturity

$12.64

Bond‑equivalent (investment) yield

5.13%

Bank discount yield (quoted)

5.00%

OutputValueUnit
Purchase price$987.36USD
Profit at maturity$12.64USD
Bond‑equivalent (investment) yield5.13%%
Bank discount yield (quoted)5.00%%
Primary result$987.36

Visualization

Methodology

Bank discount convention: T‑Bill discount yields (d) are quoted relative to face value using a 360‑day year. Price is P = F × (1 − d × (days/360)).

Bond‑equivalent (investment) yield converts the realized return to an annualized yield on an investment basis: y_BE = ((F − P) / P) × (365 / days).

When computing from a given BEY, price is computed as P = F / (1 + y_BE × (days/365)). All methods compute profit as F − P.

Worked examples

Example 1 — From discount: Face = 1000, d = 0.05, days = 91 → P = 1000 × (1 − 0.05 × 91/360) = 987.36 (approx), profit = 12.64, BEY ≈ (12.64 / 987.36) × (365/91) ≈ 0.0527 (5.27%).

Example 2 — From price: Face = 1000, P = 975, days = 182 → d = ((1000−975)/1000) × (360/182) ≈ 0.0495 (4.95%), BEY ≈ ((25)/975) × (365/182) ≈ 0.0512 (5.12%).

Example 3 — From BEY: Face = 1000, y_BE = 0.06, days = 28 → P = 1000 / (1 + 0.06 × 28/365) ≈ 995.42, profit ≈ 4.58, discount yield ≈ (4.58/1000) × (360/28) ≈ 0.0589 (5.89%).

F.A.Q.

Why are there different yields (discount yield vs bond‑equivalent yield)?

Bank discount yield is a market quoting convention based on face value and a 360‑day base. Bond‑equivalent yield reflects the investor's actual return on the cash invested and is annualized on an actual/365 basis. They are different because they use different denominators and day‑count conventions.

Which day‑count basis is used?

This tool uses the common market conventions for U.S. Treasury bills: bank discount calculations use a 360‑day base; bond‑equivalent yields use a 365‑day base for annualization.

How precise are the results?

Results are computed using standard arithmetic expressions and rounded for display. Small rounding differences may occur versus broker systems that use extended precision or different intraday conventions. See the accuracy and testing notes in citations.

Can I change the face value or days to maturity?

Yes. Face value (par) and days to maturity are editable. Use the method selector to provide the appropriate input for the chosen workflow.

Sources & citations

Further resources

Versioning & Change Control

Audit record (versions, QA runs, reviewer sign-off, and evidence).

Record ID: 3517c70d8f49

What changed (latest)

v1.0.0 • 2025-11-22 • MINOR

Initial publication and governance baseline.

Why: Published with reviewed formulas, unit definitions, and UX controls.

Public QA status

PASS — golden 25 + edge 120

Last run: 2026-01-23 • Run: golden-edge-2026-01-23

Engine

v1.0.0

Data

Baseline (no external datasets)

Content

v1.0.0

UI

v1.0.0

Governance

Last updated: Nov 22, 2025

Reviewed by: Fidamen Standards Committee (Review board)

Credentials: Internal QA

Risk level: low

Reviewer profile (entity)

Fidamen Standards Committee

Review board

Internal QA

Entity ID: https://fidamen.com/reviewers/fidamen-standards-committee#person

Semantic versioning

  • MAJOR: Calculation outputs can change for the same inputs (formula, rounding policy, assumptions).
  • MINOR: New features or fields that do not change existing outputs for the same inputs.
  • PATCH: Bug fixes, copy edits, or accessibility changes that do not change intended outputs except for previously incorrect cases.

Review protocol

  • Verify formulas and unit definitions against primary standards or datasets.
  • Run golden-case regression suite and edge-case suite.
  • Record reviewer sign-off with credentials and scope.
  • Document assumptions, limitations, and jurisdiction applicability.

Assumptions & limitations

  • Uses exact unit definitions from the Fidamen conversion library.
  • Internal calculations use double precision; display rounding follows the unit's configured decimal places.
  • Not a substitute for calibrated instruments in regulated contexts.
  • Jurisdiction-specific rules may require official guidance.

Change log

v1.0.0 • 2025-11-22 • MINOR

Initial publication and governance baseline.

Why: Published with reviewed formulas, unit definitions, and UX controls.

Areas: engine, content, ui • Reviewer: Fidamen Standards Committee • Entry ID: 16bdd4f78193