Twelve tools. Five trades. Zero guesswork.
Wire size, voltage drop, load calcs, and breaker sizing for electricians. Pipe sizing, flow rate, and tank size for plumbers. Paint, quotes, tile, waste, and board feet for painters, tilers, and carpenters.
🔌 Electrician
🚿 Plumber
🎨 Painter
◻️ Tiler
🪵 Carpenter
Planning estimates, not code sign-off
Planning estimates only, using widely-used simplified trade formulas. Not a substitute for the National Electrical Code, local plumbing/building code, or sign-off from a licensed electrician, plumber, or contractor. Always verify against current code and get a permit/inspection where required.
🔌 Wire Size Calculator
Minimum copper/aluminum gauge for a given amperage.
Wire Size Calculator
Minimum recommended gauge for a single circuit, based on NEC Table 310.16 (75°C column). Not derated for bundling or high ambient temperature.
How wire sizing works
Every wire gauge has a maximum safe current-carrying capacity — its ampacity — set by NEC Table 310.16. Undersized wire overheats under load, which is a fire risk; oversized wire just costs more. This calculator finds the smallest standard gauge whose 75°C copper or aluminum ampacity meets or exceeds your circuit's amperage.
What this doesn't account for
Real-world installations often need derating: more than three current-carrying conductors bundled in a raceway, high ambient temperatures, or long conduit runs all reduce a wire's effective ampacity below the table value. This tool gives you a starting point for a single, unbundled circuit at standard conditions — always verify against the full NEC tables or with a licensed electrician before pulling wire.
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📉 Voltage Drop Calculator
Check % drop over distance and wire gauge.
Voltage Drop Calculator
Checks % voltage drop over distance for a chosen wire gauge. NEC recommends keeping branch-circuit drop under 3%, total under 5%.
Why voltage drop matters
As current travels down a wire, resistance eats a little voltage the whole way — the longer the run and the higher the current, the more you lose. Too much drop means dimming lights, struggling motors, and inefficient equipment. NEC doesn't mandate a maximum, but recommends staying under 3% for a branch circuit and 5% total (branch + feeder combined).
The fix is usually a bigger wire
If your drop comes back high, the standard remedy is upsizing the conductor — going from 12 AWG to 10 AWG roughly halves resistance. This is especially common on long outdoor runs (detached garages, well pumps, dock power) where distance adds up fast.
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🏠 Load Calculation
Simplified NEC dwelling load estimate.
Load Calculation
Simplified NEC 220 standard method for dwelling units: general lighting + small appliance + laundry circuits, with the NEC 220.42 demand factor table applied.
The standard method, simplified
NEC 220 lays out a "standard method" for calculating a dwelling's total electrical load: general lighting at 3 VA per square foot, plus dedicated small-appliance and laundry circuits, all run through a demand factor table that recognizes not everything runs at once. This calculator applies that same demand curve — 100% of the first 3,000 VA, 35% of the next 117,000 VA — then adds your fixed appliance loads (range, dryer, water heater, etc.) at face value.
A planning tool, not a permit calculation
The real NEC method applies its own demand factors to specific appliances (ranges get their own table, for instance) rather than one flat "other loads" figure. Use this to sanity-check a service size before calling an electrician, not to size an actual panel upgrade.
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🔲 Breaker Sizing
Right breaker size for continuous/non-continuous loads.
Breaker Sizing
Continuous loads (on 3+ hours) are sized at 125% per NEC 210.20(A), then rounded up to the next standard breaker size — capped at your wire's ampacity.
Why continuous loads get the 125% rule
A "continuous load" — one expected to run 3 hours or more, like lighting, HVAC, or an EV charger — heats a breaker and wire more than an intermittent one. NEC 210.19(A)(1) and 210.20(A) require sizing the circuit at 125% of a continuous load's current to leave thermal headroom. Non-continuous loads only need to be covered at 100%.
The breaker protects the wire, not just the load
A breaker's real job is protecting the conductor from overheating — so even if your load calc says you could use a bigger breaker, it should never exceed the wire's rated ampacity. This calculator caps its recommendation there when you provide a wire ampacity limit.
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🚿 Pipe Sizing
Minimum pipe size from total fixture units.
Pipe Sizing
Simplified fixture-unit method — a planning estimate based on common UPC/IPC fixture-unit-to-pipe-size guidance, not a substitute for a full code calculation.
Fixture units, explained
A fixture unit (FU) is a standardized way to represent a fixture's demand on a water or drain system — a bathroom sink might be 1 FU, a bathtub 2 FU, a toilet 3-6 FU depending on the code and fixture type. Add up all fixture units on a branch and look up the minimum pipe size that handles that combined demand without starving fixtures at peak use.
Codes vary — this is a planning number
UPC and IPC fixture-unit tables differ slightly, and local amendments vary further. Treat this as a rough sizing check before design, not a substitute for your jurisdiction's actual plumbing code table.
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💧 Flow Rate Calculator
GPM from pipe diameter and velocity.
Flow Rate Calculator
Converts pipe diameter and flow velocity into gallons per minute.
From pipe size to gallons per minute
Flow rate depends on two things: how big the pipe's cross-section is, and how fast water is actually moving through it. This calculator converts a pipe's inside diameter and an assumed velocity into cross-sectional area, then into gallons per minute using the standard cubic-feet-to-gallons conversion (1 ft³/s ≈ 448.83 GPM).
Picking a reasonable velocity
Residential supply lines are typically designed for 4-8 ft/s — too slow wastes capacity, too fast causes noise, erosion, and water hammer risk. Drainage and gravity lines use very different velocity assumptions; this tool is built for pressurized supply-line estimates.
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🛁 Tank Size Calculator
Water heater size from household peak demand.
Tank Size Calculator
Estimates a water heater's recommended nominal tank size from peak-hour household demand and a typical 70% usable-capacity factor.
Sizing for your peak hour, not your daily average
Water heaters don't need to cover a full day's usage at once — they need to cover the busiest single hour, like a morning where two showers and a dishwasher overlap. This calculator multiplies household size by a per-person peak-hour demand figure, then divides by a "usable capacity" factor, since a tank's water cools as it's drawn down and you typically only get about 70% of nominal volume before comfort suffers.
Tankless changes the math entirely
This tool is built for storage-tank water heaters. Tankless (on-demand) units are sized by flow rate and temperature rise instead — a completely different calculation.
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🎨 Paint Quantity
Gallons needed from wall area, coats, and waste.
Paint Quantity
How much paint to buy, in gallons, rounded up to the nearest quart.
The coverage-rate math
Paint coverage is usually quoted per gallon per coat — 350 sq ft/gallon is a common industry figure for smooth walls, though textured surfaces, dark-to-light color changes, and cheaper paints can push actual coverage well below that. This calculator takes your net wall area (after subtracting doors and windows), multiplies by your number of coats, divides by coverage rate, then adds a waste allowance for cutting in and roller loss.
Buying in quarts vs. gallons
The final number rounds up to the nearest quart, since most stores sell in gallon and quart increments and you're usually better off with a little extra for touch-ups than running short mid-coat.
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🧾 Project Quote
Materials + labor + markup for a painting job.
Project Quote
Materials + labor + markup for a painting job. Use the Paint Quantity tool first to get your gallons needed.
Building a quote from real numbers
A defensible quote separates materials, labor, and margin so a client can see what they're paying for. This tool takes your paint cost (gallons × price), estimates labor hours from a productivity rate (square feet a painter typically covers per hour), multiplies by your labor rate, then applies a markup percentage on top of the subtotal to cover overhead and profit.
Productivity varies by job
150 sq ft/hour is a reasonable single-coat average for a straightforward interior wall, but trim work, high ceilings, multiple colors, and heavy prep all slow things down — adjust the productivity figure to match the actual job.
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◻️ Tile Quantity
Tiles and boxes needed for an area.
Tile Quantity
How many tiles (and boxes) to buy for an area, with a waste allowance.
From square footage to boxes
This calculator converts your tile's dimensions into square feet per tile, divides your total area by that to get a raw tile count, then adds a waste allowance for cuts, breakage, and future repairs. If you know your box count, it also tells you how many full boxes to buy — always round up, since a partial box still means buying the whole box.
Don't forget pattern and layout
A straight grid layout wastes less than a diagonal or offset pattern. Use the Waste & Overage tool alongside this one if your layout is more complex than a basic grid.
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📐 Waste & Overage
Extra material to buy by layout pattern.
Waste & Overage
Recommended extra material to buy, based on your layout pattern — more complex patterns mean more offcuts.
Why pattern changes your waste percentage
Every cut tile produces an offcut, and complex layouts need more cuts. A straight grid lay against square walls might only waste 10% to trim edges. A diagonal lay adds 45-degree cuts along every border, typically pushing waste to 15%. Herringbone and other intricate patterns can require 20% or more, since nearly every piece along an edge needs a custom cut.
Always buy from the same dye lot
Tile color can vary slightly between production runs. Buying your full estimated quantity — waste included — from a single lot upfront avoids a mismatched patch if you run short later.
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🪵 Board Feet Calculator
Lumber volume and cost from dimensions.
Board Feet Calculator
Standard lumber-volume formula: (thickness × width × length) ÷ 12, times quantity.
The board foot, explained
A board foot is a volume measurement — one board foot equals a piece of lumber 1 inch thick, 12 inches wide, and 1 foot long (144 cubic inches). Hardwood and specialty lumber are almost always priced by the board foot rather than by the linear foot, since it accounts for both thickness and width, not just length.
Nominal vs. actual dimensions
A "1×6" board is nominally 1 inch by 6 inches, but after milling it's typically closer to 3/4" × 5.5" actual. For rough-sawn or full-dimension lumber, use actual measured dimensions in this calculator rather than the nominal label for an accurate board-foot count.
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Reference & formulas
The constants and formulas behind every TradeKit calculator, with sources.
Important
- Planning estimates only, using widely-used simplified trade formulas. Not a substitute for the National Electrical Code, local plumbing/building code, or sign-off from a licensed electrician, plumber, or contractor. Always verify against current code and get a permit/inspection where required.
Electrician
| Tool | Formula / basis |
|---|---|
| Wire size | Smallest gauge with 75°C ampacity ≥ circuit amps (NEC Table 310.16) |
| Voltage drop | Vd = (2 × K × I × D) / CM — K=12.9 (Cu) / 21.2 (Al) ohm-cmil/ft |
| Load calculation | 3 VA/sqft lighting + 1500 VA/circuit (appliance, laundry), NEC 220.42 demand factors |
| Breaker sizing | Continuous loads × 125% (NEC 210.20(A)), rounded to next standard breaker, capped by wire ampacity |
Plumber
| Tool | Formula / basis |
|---|---|
| Pipe sizing | Minimum pipe size from total fixture units (simplified UPC/IPC table) |
| Flow rate | GPM = π×(d/2/12)² × velocity(ft/s) × 448.83 |
| Tank size | Peak-hour demand ÷ usable capacity factor (~70% of nominal tank volume) |
Painter
| Tool | Formula / basis |
|---|---|
| Paint quantity | (Net wall area × coats) ÷ coverage rate, + waste %, rounded up to nearest quart |
| Project quote | (Material cost + labor hours × rate) × (1 + markup %) |
Tiler
| Tool | Formula / basis |
|---|---|
| Tile quantity | Area ÷ tile area × (1 + waste %), rounded up; ÷ tiles/box for box count |
| Waste & overage | Pattern-based %: straight 10%, large-format 12%, diagonal 15%, herringbone 20% |
Carpenter
| Tool | Formula / basis |
|---|---|
| Board feet | (thickness in × width in × length ft) ÷ 12, × quantity |