Free, accurate calculators built around the standards marine surveyors and competent boat builders actually reference — primarily ABYC E-11. We use these on Yoto. They don't track you, they don't gate behind email signups, and they don't pretend automotive wire is okay for boats.
Electrical Calculators
Marine Wire Gauge Calculator (ABYC E-11)
Pick the right AWG for any DC circuit on your boat. Handles voltage drop and ampacity in one calculation, supports 12V/24V/32V/48V systems, and tells you which constraint actually drove the answer (almost always voltage drop, not ampacity).
Marine Fuse Sizing Calculator (ABYC E-11)
Sizes the right fuse for any load — continuous, intermittent, motor, or inverter — using ABYC's 125%/156% rules. Optional wire-size check warns you if the recommended fuse can't be safely protected by the wire you've run. Includes Class T recommendations for lithium banks.
Battery Bank Sizing Calculator
Plug in daily Wh consumption, days of autonomy, and battery chemistry. Get the right bank size in Ah at your chosen system voltage. Handles LiFePO4, AGM, gel, and flooded lead-acid with the right depth-of-discharge for each chemistry.
Solar Sizing Calculator
Daily energy need + peak sun hours + realistic loss factors → recommended panel wattage. Tuned to what we actually see on Yoto in the tropics, with honest derating for marine reality (shading, MPPT efficiency, panel temperature).
Other Tools We Recommend
Tools we didn't build but use ourselves and trust enough to send you to:
Catamaran Haul Out Map
Interactive map of every boat yard on the US East Coast, Gulf Coast, Bahamas, and Caribbean that can actually haul a wide-beam catamaran. Filterable by beam, mast height, and displacement. Built by The Cruisers Company. If you have a cat over 18 feet wide, this saves you hours of phone calls — most travel lifts can't fit you, and this tool tells you which ones can before you arrive.
Coming Soon
- Voltage drop reverse calculator — pick a wire size, see actual drop at a given current and length
Why These Exist
Most marine calculators online are wrong, gated behind ads, or quietly using automotive standards instead of marine ones. We built ours because we needed them and couldn't trust what was out there.
If you find a bug or want a calculator built, email braden@goodkit.io.
About the Standards
ABYC E-11 ("AC and DC Electrical Systems on Boats") is the rule book the marine industry follows for boat electrical work. It's what surveyors check, what insurance companies expect, and what ABS, Lloyd's, and ABYC-certified technicians are trained on. These calculators reference E-11 directly for resistance values, ampacity tables, voltage drop limits, and overcurrent protection sizing.
For RV applications, the relevant standards differ (NEC Article 551 and RVIA codes) but the underlying physics and many of the practical rules are similar.