Conservation of mass, applied to a kitchen
A range hood is a pump. Whatever it sends up the duct leaves the house, and an identical volume has to enter the house, because the house is not expanding. There is no version of this where the air does not come back. The only question is which path it uses and what pressure difference it takes to drive it through that path.
In a leaky house the answer is boring. Six hundred CFM spread across thousands of small gaps in the envelope arrives at a pressure difference of a pascal or two, which is less than a gust of wind, and nothing observable happens. In a tight house the same six hundred CFM has far fewer paths, so the pressure difference required to drive it through them is much larger, and the house hunts for the easiest opening it can find.
How the arithmetic works
Envelope leakage is characterised by a blower door test, which measures the flow needed to hold the house at fifty pascals of pressure difference. That gives a CFM50 figure, or an ACH50 figure once you divide by the house volume and multiply by sixty.
Leakage does not scale linearly with pressure. It follows a power law: flow is proportional to pressure raised to an exponent that real envelopes put somewhere between about 0.6 and 0.7. So the flow available at some other pressure is the CFM50 multiplied by the ratio of that pressure to fifty, raised to the exponent.
Run it for a house at seven air changes at fifty pascals with twenty thousand cubic feet of volume. CFM50 is 20,000 times 7 divided by 60, which is about 2,330. At three pascals that envelope supplies 2,330 times (3/50) to the power 0.65, which comes to roughly 375 CFM. A 600 CFM hood plus a dryer is asking for more than double that, so the house either goes considerably more negative than three pascals or gets the difference from a deliberate opening.
Why depressurisation is the safety question and not a comfort one
Most of what a negative house does is harmless and annoying: a door that swings, a whistle at a window, air pulled in from the crawlspace that smells like the crawlspace. One consequence is not in that category, and it is worth being direct about.
An atmospherically vented fuel-burning appliance relies on hot gas rising up a flue under its own buoyancy. That draft is weak. If the house around it is pulled negative hard enough, the flue becomes just another opening through which the house can draw replacement air, and the flow reverses. Combustion products, carbon monoxide among them, then enter the living space instead of leaving it. This is a documented failure mode with a known cause and it does not announce itself.
What follows from that is not a procedure. It is that a house with an atmospherically vented water heater, furnace, boiler, wood stove or open fireplace and a large exhaust appliance is a combination that needs to be assessed by someone licensed, with instruments, on that specific house. A carbon monoxide alarm is worth having in any case and does not answer the question, because it tells you about a failure that has already happened. Sealed combustion and direct vent appliances draw their air from outside and do not have this vulnerability, which is one reason they exist.
What makeup air actually involves
The quantity is the easy part and this page gives it to you. The system is not just a duct. A hole in the envelope large enough to feed a powerful hood is a hole that is there in January, so a real makeup air arrangement has a damper that opens when the hood runs and closes when it does not, some means of interlocking the two, and frequently a way of tempering the incoming air so that opening it does not deliver outdoor air at outdoor temperature into the kitchen at high speed.
Where it terminates matters as well. Air pulled from an attached garage, a crawlspace or near a flue is worse than no plan at all. Sizing the opening tells you the scale of the problem, which is often the useful part: seeing that a hood needs a ten inch dedicated opening tends to change the conversation about which hood to buy.
Reading this alongside the rest of the system
The hood CFM you enter should be a flow rate on the duct you actually have, not the number on the carton. Hood ratings are commonly quoted at zero static pressure, and a long run with a spring-loaded wall cap takes a substantial bite out of it. The duct size calculator covers sizing that run and the flex duct penalty calculator covers what happens if any of it is flexible.
The same balance question shows up at a smaller scale elsewhere in the house. The bathroom fan sizing calculator handles a much smaller exhaust on a much longer duct, the whole house ventilation calculator covers continuous rather than intermittent ventilation, and the air sealing payback calculator is where the envelope tightness number in this calculation comes from and goes to. Tightening a house and adding a big hood are decisions that interact, and doing one without thinking about the other is how houses end up needing this page.
Questions people ask
Does my range hood need makeup air?
That depends on the exhaust rate, on how tight the house is, and on what your jurisdiction requires, and only the first two are arithmetic. This page gives you the quantity side: how much air the envelope can supply at a pressure difference you choose, and how much the hood asks for beyond that. The requirement side comes from the code your area has adopted and from the official enforcing it, and those differ between places and between editions, so no web page can tell you whether you need it. What the arithmetic will tell you is whether it is close. If a hood is asking for several times what the envelope supplies at a small pressure difference, the answer is going to be yes on physics regardless of what any rule says.
What is a blower door number and can I estimate it instead?
A blower door is a calibrated fan in a doorway that pressurises or depressurises the house to fifty pascals and measures the airflow required. That flow is the CFM50, and dividing it by house volume and multiplying by sixty gives ACH50. You can estimate it from the age and construction of the house, which is what the tightness selector on this page does, and the estimate has a wide error band — houses of similar vintage routinely differ by a factor of two or three. Because the whole calculation is sensitive to this input, run it at both ends of a plausible range and see whether the conclusion is stable. If it is not, the estimate is not good enough for the decision and a real test is what you need.
Can I just crack a window when the hood is on?
It supplies air, in the sense that an open window is a large hole, and people do it. As a system it has obvious weaknesses: it depends on someone remembering every time, it is unpleasant in cold weather so it will stop happening, the window may not be in a location that keeps the incoming air away from an appliance flue, and none of it satisfies anything that might be required in your jurisdiction. It is also worth noting that an open window near the hood can short-circuit the hood itself, drawing outdoor air straight past the range and up the duct without ever capturing much from the cooktop. If the house genuinely needs makeup air, an interlocked and properly located provision is a different thing from a window, and the reason it exists is that the window does not reliably work.
Is a downdraft or a recirculating hood a way around this?
A recirculating hood exhausts nothing, so it creates no pressure imbalance and this entire calculation does not apply to it. What it also does not do is remove moisture, heat or combustion products from cooking; it passes air through a filter and returns it to the kitchen. That is a genuine trade rather than a trick. A downdraft is a ducted exhaust like any other and counts fully in the total on this page, and downdrafts often run at high flow rates because they are fighting the natural tendency of hot air to rise. Enter one in the other exhaust field or as the hood itself, whichever it is.
How do I know whether a specific house has a backdrafting risk?
By having it assessed by someone qualified, on site, with instruments. This is not a calculation and not something to determine from a web page, and the reason is that the answer depends on the specific appliances, their venting, the condition of the flue, the layout of the house and the pressures actually achieved with the exhaust running. What is worth knowing generally is which appliances are exposed: atmospherically vented ones, which draw combustion air from the room and rely on natural draft up a flue, are the vulnerable category. Sealed combustion and direct vent appliances take their air from outdoors through a dedicated pipe and are not exposed in the same way. If you have the former and are adding significant exhaust, that is the trigger for a professional assessment, and a carbon monoxide alarm is a sensible thing to own regardless without being an answer to the question.