
The simplest way with irregular income is to turn it into a steady paycheck and let a buffer absorb the swings. Add up your required monthly bills and essentials to get your monthly nut, then set a conservative salary that you can hit in lean months, like the average of your worst three months or about 80 percent of your 12 month average. Funnel all client payments into an income holding account, keep a regular checking account for bills, and have a separate savings for taxes plus an emergency fund. If you hate multiple accounts, at least keep taxes separate, and use bank sub-savings buckets for emergency and irregular bills if your bank supports it.
On every deposit, pay the buckets first. Move 30 to 35 percent to the Tax account right away, then 10 percent to the Emergency fund until you hit 3 to 6 months of expenses, and 5 to 10 percent to an Irregulars bucket for things like insurance premiums, software renewals, car repairs, and annual fees. Leave the rest in the holding account as your buffer. Set an automatic transfer from holding to checking on the 1st of the month or on the 1st and 15th for the fixed salary you chose. In good months the buffer grows toward 1 to 3 months of your salary. In lean months the buffer covers your paycheck so the bills still get paid on time.
For US taxes, make estimated payments from the Tax account four times a year, typically by April 15, June 15, September 15, and January 15, and adjust the tax percentage if you have large deductions or a high state rate. Track business expenses and pay them from the holding account before calculating taxes, since your tax is on profit, not gross. Do a 10 minute weekly check to clear new deposits, confirm the automatic transfers, and glance at bucket balances, and do a short quarterly review to tweak the salary and percentages if your average changes.