Ethernet Cable Calculator

bit time = 1/rate; propagation delay ≈ 5 ns/m (NVP ≈ 0.66). Standard segment limit is 100 m.

When you need it: Checking maximum cable reach, pair-to-pair delay skew and PoE voltage drop for 10/100/1000BASE-T runs before committing a cable spec.

Worked example: A Cat5e 100BASE-TX channel is capped at 100 m (90 m solid horizontal + 10 m patch); 1000BASE-T uses all four pairs. The loop resistance of 100 m of Cat5e is about 2 × 9.38 = 18.8 Ω, so PoE at 0.36 A drops roughly 0.36 × 18.8 ≈ 6.8 V along the run.

Tips & gotchas:

• The 100 m channel limit holds regardless of speed — it's a timing/attenuation budget, not just resistance.
• For Gigabit keep pair-to-pair delay skew under ~50 ns; the PHY de-skews the four pairs but only within a window.
• Use Cat6/6A for 10GBASE-T, and keep bundles loose to limit alien crosstalk at 10G.
• Size PoE for worst-case cable resistance and temperature; copper resistance rises ~0.4%/°C.