Calculator Group
Home Systems Sizing
Published July 19, 2026
The 12 calculators in this group
- Heat Loss CalculatorDesign heat loss summed across walls, ceiling, floor, windows and air infiltration, so you can see which one is actually costing you.
- Room AC BTU CalculatorRoom air conditioner sizing from floor area, adjusted for ceiling height, sun exposure and occupants — the corrections that matter.
- Duct Size CalculatorDuct diameter from the airflow a room's heat load needs and a target velocity, with the velocity you actually get in the size you can buy.
- Water Heater Size CalculatorPeak-hour hot water demand added up fixture by fixture, converted into a tank size and the flow and BTU a tankless unit would need.
- Generator Sizing CalculatorRunning watts plus the largest single motor surge, not the sum of every surge — the difference between a 5 kW rental and a 12 kW one.
- EV Charger Circuit CalculatorBreaker size, minimum copper conductor and voltage drop for an EV charger circuit, treating it as the continuous load it is.
- Solar Array Size CalculatorSystem size and panel count from annual kWh, peak sun hours and a real derate factor, with the roof area that many panels need.
- Radiant Floor Loop CalculatorPEX length from tube spacing, split into loops that respect the maximum length for your tube size, with manifold ports counted.
- Attic Ventilation CalculatorNet free ventilating area for an attic at the 1:150 or 1:300 ratio, split into balanced intake and exhaust and converted to vent lengths.
- Bathroom Fan CFM CalculatorFan CFM by both the air-change and fixture methods, plus the equivalent duct length that decides whether the fan can deliver it.
- Recessed Lighting Layout CalculatorFixture count from both the ceiling-height spacing rule and the lumen method, taking whichever is larger, with the wall offset.
- Firewood Cord CalculatorFull cords and face cords from a stack's dimensions, showing exactly how many face cords of your log length make one real cord.
Nothing in this group is sized to what a house typically uses, and that is precisely why the numbers this group produces tend to look bigger than intuition expects. A furnace is sized to the coldest design day the local climate throws at it, not an average January afternoon. A water heater is sized to the busiest single hour a household actually has — two showers and a dishwasher running at once — not an average hour spread across the day. A generator is sized around the single largest piece of equipment's starting surge, a load that might exist for less than a second when a compressor kicks on, rather than the steady running total of everything plugged in.
Sizing for peak rather than average is not caution for its own sake — undersizing any of these means the system fails at exactly the moment it's needed most, the coldest night, the busiest morning, the outage that lasts a week. The trade-off this group asks the reader to accept is that a correctly sized system looks over-built against a typical day, because a typical day was never the day it was sized for.
Every calculator in this group sizes for the worst moment, not the average one
Heat loss is calculated against a design temperature specific to the local climate — a realistically cold night the area actually experiences, not the average winter temperature, because a furnace only earns its keep on the coldest nights, and being merely adequate on an average day is not the test that matters. Duct sizing follows directly from that same peak load, since the ductwork has to move the air a system delivers at its maximum output, not its typical cycling rate.
Firewood is the exception worth naming honestly: a cord calculation is a fuel-volume conversion against total heating demand over a season, not a peak-moment sizing question the way the electrical and mechanical calculators in this group are. It sits here because it answers a heating question, not because it shares the peak-versus-average shape with the rest of the group.
A generator's size comes from one appliance, not the sum of everything running
It's tempting to size a generator by adding up the running wattage of everything it needs to power, and that total understates the real requirement, because motors and compressors draw a starting surge well above their running wattage for a brief moment when they switch on. The number that actually sets generator size is the running load of everything else, plus the single largest starting surge among the equipment that might start while everything else is already running — not the sum of every appliance's own surge, since surges don't all happen simultaneously in practice.
That's a genuinely different calculation than most people expect walking in, and it's why two households with an identical total running wattage can need differently sized generators if one has a well pump or an air conditioner compressor with a much larger starting surge than the other's equipment produces.
Anything touching the electrical panel here is a sizing aid, not a permit
Generator, EV charger circuit and solar array sizing all involve calculations a licensed electrician makes as part of a real installation — breaker sizing, conductor gauge, voltage drop over distance, and confirming the panel actually has capacity for a new circuit without exceeding its rating. This site works through that arithmetic honestly and completely, but a correct calculation on a page is not a substitute for a licensed electrician pulling a permit and inspecting the installed result, and treating it as one is where a sizing exercise turns into a code violation or a fire risk.
The distinction that matters practically: everything in this group is useful for understanding what a job actually requires before calling a professional, so the conversation starts from an informed position rather than a blank one. None of it is useful as a substitute for the professional the job requires, and the calculators that touch the panel say so plainly rather than leaving that caveat implied.
The order that avoids sizing equipment to a building that changes underneath it
Envelope decisions come before equipment sizing, not after. A building's insulation level directly sets its heat loss, and heat loss is the number that sizes the furnace or heat pump — so improving insulation after a system is already sized means the equipment is now oversized for the building it's heating, running short cycles that wear it out faster without actually heating any better. The BTU sizing guide and the R-value by climate zone guide both sit upstream of the equipment-sizing calculators for exactly this reason.
Once the envelope is settled, BTU sizing and duct sizing follow from it directly, and only after those are settled does it make sense to size a generator or a solar array against the resulting electrical or heating load — sizing backup power or renewable generation before the load it needs to cover is finalized means resizing it once the real number is in.
Ventilation and lighting are sized against two rules at once, not one number
Attic ventilation has to satisfy a net free area requirement split between intake and exhaust, not just a single total — an attic with plenty of exhaust vents but not enough intake near the eaves does not ventilate correctly even if the combined total looks adequate on paper, because air has to actually flow from one to the other rather than just exist in both places.
Recessed lighting works against two constraints simultaneously in a similar way: a spacing rule that keeps fixtures from being too far apart for even coverage, and a total lumen target for the room's actual size and use. A layout that satisfies the spacing rule with too few fixtures under-lights the room even though the spacing looks correct, and a layout that hits the lumen target with fixtures crowded too close together creates hot spots instead of even light — both rules have to be satisfied together, and neither one alone is sufficient.
FAQ
Do I need the heat loss calculator before the BTU calculator, or are they answering the same question?
Heat loss first — it's the more detailed, envelope-specific calculation, working from actual wall, window and ceiling construction to a design-day loss figure in BTU per hour. The BTU calculator is a faster sizing pass for straightforward cases, useful when a full envelope breakdown isn't practical, but where the two disagree, the heat loss figure built from the actual construction is the one worth trusting.
Does the generator calculator replace the need for an electrician's load calculation?
No. It gives an honest running-plus-surge wattage figure to walk into that conversation with, which is genuinely useful, but the electrician's load calculation covers the panel's actual capacity, code-required transfer switch details, and the specific circuits being backed up — details this page does not have visibility into and does not attempt to substitute for.
Should duct sizing be run before or after the BTU calculator?
After. Duct sizing works from the system's actual output airflow, which is a function of the equipment size the BTU or heat loss figure determines, so there's nothing for the duct calculation to size against until the equipment side is settled first.
Is the EV charger calculator enough to tell me whether my panel has room for a new circuit?
It tells you what the new circuit itself needs — breaker size, conductor gauge, voltage drop at the run length involved — which is real and useful information. It does not tell you what your panel's existing total load already is, and that's the number that actually determines whether there's room, which is a measurement an electrician takes at the panel itself rather than something this calculator can see from a circuit-level input.