California Form 5805 is the worksheet you use to calculate the penalty for underpaying estimated state income tax, and to reduce it when the standard method overstates what you actually owe. The Franchise Tax Board charges interest on any shortfall in each installment period, so a return that lands on the right total for the year can still generate a penalty if the payments arrived on the wrong schedule. The form handles two jobs: it walks the period-by-period math, and it gives you the tools to lower the number through the annualized income method or a waiver.
Who Actually Owes a Penalty
You only need to think about Form 5805 if you were required to make estimated payments in the first place. That means expecting to owe $500 or more in California tax after withholding and credits ($250 if married or in a registered domestic partnership filing separately).1Franchise Tax Board. Estimated Tax Payments
Even above that threshold, you avoid the penalty entirely if your combined withholding and estimated payments hit one of two safe harbors:
- At least 90% of the tax on your current year’s return, or
- At least 100% of the tax on your prior year’s return, provided that return covered a full 12 months.1Franchise Tax Board. Estimated Tax Payments
Higher earners get a tougher version of the prior-year test. If your California adjusted gross income exceeded $150,000 in the prior year ($75,000 for married/RDP filing separately), the prior-year safe harbor rises to 110% of last year’s tax.2Franchise Tax Board. 2025 Instructions for Form 540-ES Estimated Tax for Individuals – Section: C. Limit on the Use of Prior Year’s Tax At $1,000,000 or more of AGI, the prior-year option disappears altogether. You must pay 90% of current-year tax; there is no fallback to a percentage of last year’s bill.
California’s 30/40/0/30 Installment Schedule
The single most common reason people end up on Form 5805 is that California’s estimated tax schedule is not the federal schedule. Federal payments come in four equal 25% installments. California is front-loaded:3Franchise Tax Board. 2026 Instructions for Form 540-ES Estimated Tax for Individuals
- First installment (April 15): 30% of the required annual payment
- Second installment (June 15): 40%
- Third installment (September 15): nothing due
- Fourth installment (January 15 of the following year): 30%
Seventy percent of your annual amount is due by June 15. If you carried a federal four-equal-payments habit into California, you have almost certainly underpaid the April and June installments, and you cannot cure it by overpaying in September or January. The penalty is calculated period by period, so hitting the correct annual total is not enough.
How the Penalty Is Calculated
Form 5805 first determines your required annual payment using the safe harbor rules above. It then splits that annual amount across the four periods on the 30/40/0/30 schedule and compares each required installment to what you had actually paid by that period’s due date. Any shortfall becomes the underpayment for that period.4Franchise Tax Board. 2025 Instructions for Form FTB 5805
The penalty on each shortfall is an interest charge, not a flat percentage. Interest starts the day after the installment due date and stops when the shortfall is made up, either by a later estimated payment or by the payment you send with your return. Missing April costs more than missing January because the money is outstanding longer.
The FTB sets the rate semi-annually under a formula tied to federal short-term rates.5California Legislative Information. California Revenue and Taxation Code RTC 19521 For 2026, the personal income tax underpayment rate is 7% for both halves of the year.6State of California Franchise Tax Board. Interest and Estimate Penalty Rates The form has you count days between each installment date and the date the shortfall was covered, apply the rate, and total the four periods.
Do You Have to File the Form Yourself?
Not always. The FTB is willing to run the standard calculation and bill you directly. You should complete Form 5805 proactively when you want to claim something the FTB will not give you on its own: a lower number under the annualized income method, or a waiver of the penalty. Without the form, the standard method stands and the bill reflects it.4Franchise Tax Board. 2025 Instructions for Form FTB 5805
Reducing the Penalty With the Annualized Income Method
The standard calculation assumes your income arrived in even quarters. If it did not, the standard method can charge you for failing to pay tax on money you had not yet received. A large Q4 capital gain, a December bonus, a business sale late in the year: these are the classic cases.
The annualized income installment method recalculates your required installment for each period based only on the income you had actually earned through the end of that period. California builds this calculation into Part III of Form 5805 itself, on Sides 3 and 4. There is no separate schedule to attach.4Franchise Tax Board. 2025 Instructions for Form FTB 5805
For each of the four periods, you calculate tax on the income earned through that period’s cutoff, apply the annualization factor for that period, and compare the resulting installment to the standard one. The form takes the smaller of the two for each period, so the method can only help, never hurt.
One catch: if you use the annualized method for any period, you must use it for all four. You cannot pick the periods where it lowers the number and revert to the standard method elsewhere. You also have to check the annualized income box on the corresponding line of your main return (Form 540 line 113, Form 540NR line 123, or Form 541 line 45) so the FTB knows to expect Part III.4Franchise Tax Board. 2025 Instructions for Form FTB 5805
Waivers That Cancel the Penalty
Even when the math on Form 5805 produces a positive number, the FTB may waive the penalty in two situations:7Franchise Tax Board. 2025 Instructions for Form FTB 5805 – Section: C. Waiver of the Penalty
- Casualty, disaster, or unusual circumstance where imposing the penalty would be inequitable. A house fire that destroyed financial records or a medical emergency that prevented timely payment are the kinds of facts the FTB is looking at.
- Retirement after age 62 or becoming disabled during the current or prior tax year, where the underpayment was due to reasonable cause rather than neglect.
You request the waiver in writing, attached to Form 5805, with a narrative explaining what happened and why it prevented timely payments. A checked box alone will not do it.
Disaster Area Postponements
Separately from the Form 5805 waiver, the FTB can postpone filing and payment deadlines for taxpayers in a federally or state-declared disaster area.8State of California Franchise Tax Board. Emergency Tax Postponement When that happens, the postponed date replaces the original installment due date for penalty purposes. Check the FTB’s disaster relief page before you calculate the penalty, because a postponement may eliminate the underpayment on its own.
If You Farm or Fish
Form 5805 is not the right form if at least two-thirds of your gross income (current or prior year) comes from farming or fishing. You use Form 5805F instead, and you qualify for a simplified estimated tax process: pay everything in a single installment by January 15 of the following year, or file the return by March 1 and pay in full, in which case no estimated payments are required at all.9Franchise Tax Board. 2025 Instructions for Form FTB 5805F Underpayment of Estimated Tax by Farmers and Fishermen3Franchise Tax Board. 2026 Instructions for Form 540-ES Estimated Tax for Individuals The casualty and retirement/disability waivers work the same way on 5805F.
Filing and Paying
Attach the completed Form 5805 to the back of your California return: Form 540, Form 540NR for nonresidents, or Form 541 for estates and trusts.10California Franchise Tax Board. Form 5805 – Underpayment of Estimated Tax by Individuals and Fiduciaries If you e-file, your software carries the 5805 data into the electronic return. The penalty amount flows to the designated line on your main return and increases your total tax due, which you pay along with any remaining balance.
If the FTB has already sent a penalty notice and you believe the annualized method or a waiver produces a lower number, file Form 5805 with an amended return to claim the reduction. The form is the proof. Without it, the standard calculation is what the FTB will keep.