4-Week T-Bill Calculator
Calculate the exact purchase price, earnings, and annualized yield for a 4-week (28-day) Treasury Bill. The duration is fixed at 28 days — just enter your face value and current discount rate.
Minimum $100 — T-Bills are sold in $100 increments on TreasuryDirect.
Use the latest 4-week T-Bill auction rate from TreasuryDirect.gov.
Fixed at 28 days (4 weeks) — auctioned every Tuesday.
What is a 4-Week T-Bill?
A 4-week Treasury Bill is the shortest standard T-Bill duration available, maturing in exactly 28 days. It is auctioned every Tuesday by the U.S. Treasury and is one of the most liquid short-term government securities available to individual investors.
Like all T-Bills, the 4-week bill is sold at a discount to its face value. You pay less than face value today and receive the full face value at maturity 28 days later. The difference is your earnings.
4-Week T-Bill Formula
- Discount = Face Value × (Discount Rate / 100) × (28 / 360)
- Purchase Price = Face Value − Discount
- Annualized Yield = (Discount / Purchase Price) × (365 / 28) × 100
Face Value: $10,000 | Duration: 28 days
Discount = $10,000 × 4.5% × (28/360) = $35.00
Purchase Price = $10,000 − $35.00 = $9,965.00
Annualized Yield = ($35 / $9,965) × (365/28) × 100 = ~4.58%
Who Should Use the 4-Week T-Bill?
The 4-week T-Bill is best for investors who need maximum flexibility and liquidity. Because it matures every 28 days, you can reassess your options monthly and reinvest at whatever rate the next auction produces.
- Money you may need within the next month
- Investors in rising rate environments — roll over frequently at higher rates
- Businesses parking payroll or operational cash temporarily
- First-time T-Bill investors testing the process with a short commitment
4-Week vs Longer Duration T-Bills
| Duration | Days | Yield (typical) | Liquidity | Best For |
|---|---|---|---|---|
| 4-Week | 28 | Moderate | ⭐⭐⭐⭐⭐ | Max flexibility, rising rates |
| 13-Week | 91 | Moderate-High | ⭐⭐⭐⭐ | Quarterly planning |
| 26-Week | 182 | High | ⭐⭐⭐ | Best balance — most popular |
| 52-Week | 364 | Highest (normal) | ⭐⭐ | Lock in rate for a year |
Not sure which duration is right for you? Read our Best T-Bill Duration Guide for a full breakdown.
Tax Treatment of 4-Week T-Bills
Like all T-Bills, earnings from a 4-week T-Bill are subject to federal income tax but are exempt from state and local taxes. This makes them more attractive than CDs or savings accounts for investors in high-tax states like California, New York, or New Jersey.
See your real after-tax yield based on your federal bracket and state.
Frequently Asked Questions
Are 4-week T-Bills safe?
Yes — 4-week T-Bills are backed by the U.S. government and are considered one of the safest investments in the world. If held to maturity, you are guaranteed to receive the full face value. There is no credit risk.
How much can I earn from a 4-week T-Bill?
At a 4.5% discount rate, a $10,000 4-week T-Bill earns approximately $35 over 28 days, with an annualized yield of about 4.58%. Returns are paid as a lump sum at maturity — not as monthly interest.
Why invest in 4-week T-Bills?
4-week T-Bills are ideal for maximum flexibility. They mature every 28 days, letting you reinvest at higher rates if rates are rising, or redirect cash quickly if you need it. They are also popular for businesses parking short-term operational cash.
Are 4-week T-Bills exempt from state tax?
Yes — like all U.S. Treasury securities, 4-week T-Bill earnings are exempt from state and local income taxes. You only pay federal income tax on the earnings. This makes them more attractive than CDs or savings accounts in high-tax states like California or New York.
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