One founder quits her job and goes all-in. The other keeps his salary "until we get traction." Both are sincere. Both are contributing. And in eight months, this exact setup will produce the sentence I heard in more case files than any other: he is not pulling his weight, and he owns half the company.
The part-time cofounder question is not really about a percentage. It is about pricing risk honestly while the relationship can still afford honesty.
What equity actually pays for
Equity does not pay for talent. Talent can be salaried. Equity pays for risk and for the years of future work the company cannot pay cash for. A part-time cofounder, by definition, is taking materially less of both: the salary keeps running, the downside is cushioned, and the hours are a fraction.
A part-time cofounder is not half a founder. But less risk and fewer hours earn a different deal, and both of you already know it. The split that pretends otherwise does not buy harmony; it buys resentment on an installment plan, paid by the full-time founder every single morning.
So yes, a meaningful discount against the full-time founder is normal. Resist the urge to ask for a magic number, though. The honest range depends on three things you can actually reason about: how many hours, for how long, and how cushioned the downside really is. A founder at thirty hours a week with a firm date to go full-time is a different animal from a founder at eight hours a week indefinitely. Price the reality in front of you, not a blog post's percentage.
The transition date is the real clause
Most part-time arrangements are sold internally as temporary: "I'll go full-time at funding, at revenue, in January." Fine. Then write exactly that, because this is where the arrangement lives or dies. Three elements:
A defined trigger. A date, or an objective milestone (closed round, revenue threshold). "When it makes sense" is not a trigger; it is a future argument.
What happens at the trigger. The equity story can change when the commitment changes: a step-up in the split, or an accelerated vesting rate from that point. Going full-time should be worth something, on paper.
What happens if the trigger passes and nothing changes. This is the clause nobody wants to write and everyone ends up needing. If the transition never happens, the deal must adjust without requiring a confrontation: the part-time founder's unvested portion recalibrates, or a buyback at a pre-agreed formula kicks in. Written while everyone is friends, this is arithmetic. Improvised later, it is the dispute.
Vesting does the heavy lifting
Whatever numbers you choose, put everyone on a vesting schedule with a cliff, and consider tuning it to commitment: the part-time founder's equity can vest proportionally to time actually contributed, or on a longer schedule until the transition. Vesting converts the unanswerable question "what will he deserve?" into the answerable one "what has he earned so far?". That conversion is most of the fight, removed in advance.
Before incorporation, all of this governs intended equity, which is exactly why now is the moment: recalibrating a promise costs an amendment; recalibrating issued shares costs a lawyer, a valuation argument and, too often, the friendship.