Every consultant eventually asks this in a mild panic, usually right after a strong quarter. The honest answer is a range with reasons attached, and a system that makes the range irrelevant.
Why 25-30% of net profit is the starting point
For most marketing consultants, setting aside 25-30% of net profit, revenue minus business expenses, covers the federal bill with room to spare at moderate income levels. The floor is set by the stack: 15.3% self-employment tax applies from nearly the first dollar of profit, and federal income tax layers on top at your marginal bracket. At lower profit levels, the standard deduction, the deduction for half your self-employment tax, and the qualified business income deduction soften the income tax side, which is why the blended rate lands well below what the bracket table suggests. As profit climbs into six figures, your marginal bracket rises and the blended rate follows, pushing high earners toward 30-35% or beyond. State income tax is the other big swing: a consultant in a no-income-tax state can hold the low end, while one in a high-tax state should add several points. Two things the percentage must be applied to correctly: net profit, not gross revenue, and every payment, not just the ones that feel like profit.
Making the percentage automatic per payment
The savings rate only works as a reflex. The moment a client payment lands, move the chosen percentage into a separate account, ideally at a different bank, labeled taxes, before the money registers as spendable. Consultants with retainers can automate it as a recurring transfer sized to the retainer; project-based consultants move it manually the day the invoice clears. That account then funds your four estimated payments, due April 15, June 15, September 15, and January 15, and whatever remains after the annual return is your buffer or a head start on next year. What breaks the system is exceptions: skipping the transfer during a slow month, borrowing from the tax account for a laptop, or applying the rate only to big invoices. The rate is a policy, not a mood.
Tuning the number to your actual situation
The 25-30% band is a starting point, not a verdict, and three adjustments refine it fast. First, look backward: after your first full year, your actual effective rate, total tax divided by net profit, is a far better guide than any rule of thumb, so recalibrate to it. Second, count your deductions honestly: software subscriptions, contractors, courses and certifications, home office, and your own marketing all shrink profit before the rate applies, so consultants with thin expenses need a higher rate on a bigger base. Third, entity changes move the math: an S corp election reduces the self-employment tax layer and shifts part of your bill into payroll withholding, which usually lowers what the savings account must absorb. When in doubt for a season, hold the higher end of the range. The failure mode of oversaving is a pleasant refund-sized cushion in May; the failure mode of undersaving is borrowing to pay the IRS.