An extension to file is not an extension to pay tax.
Idaho grants an automatic 6-month extension with no written request, provided the taxpayer pays by the original due date either 80% of the estimated current-year tax or 100% of the total tax shown on the prior year's return (if one was filed). Interest still accrues on any unpaid tax regardless of which threshold is met.
| Item | Detail |
|---|---|
| Form required | None - no written extension request is required; Form 51 is a payment voucher only |
| Extension length | 6 months, to October 15, 2026 |
| Payment threshold | 80% of the estimated current-year tax, or 100% of the prior year's total tax (if a return was filed), paid by the original April 15 due date |
| Authority | Idaho Code §63-3033 (extension of time) |