Add the conductors you are actually pulling — any mix of sizes and insulation types — and get the fill percentage, the allowable area, a pass/fail verdict and the smallest conduit that passes. Or ask the reverse: how many of one size fit, with the rounding rule applied and flagged. The chart below is computed from the dimensions at load time, and everything runs in your browser.
Maximum number of THHN/THWN-2 conductors of one size per trade size of conduit, at the 40% limit (53% where the answer is one conductor, 31% where it is two), with the 0.8 rounding rule applied. Every cell is calculated on this page from the conduit and conductor areas when it loads, not typed in; dotted cells are the ones where the rounding rule added a conductor. Switch conduit type with the tabs.
Conduit fill is the sum of the cross-sectional areas of the conductors, including insulation, divided by the total internal area of the conduit. The limit depends on how many conductors are in the raceway:
Example, 3/4 in EMT (0.533 in² internal): five conductors are allowed 0.40 × 0.533 = 0.2132 in² between them. Three #8 THHN (3 × 0.0366) and two #10 THHN (2 × 0.0422) total 0.1520 in²: 28.5% of the conduit and 71.3% of the allowance. It passes.
Example: 3/4 in EMT, #14 THHN. Allowable 0.2132 in² ÷ 0.0097 in² = 21.98, decimal .98 ≥ .8, so the answer is 22, not 21. With #12 THHN in the same conduit, 0.2132 ÷ 0.0133 = 16.03 and the answer is 16. The rule applies only to a run of same-size conductors; a mixed run is a plain area comparison and never rounds up.
1/2 in EMT holds 12 × #14, 9 × #12 or 5 × #10 THHN; 3/4 in EMT holds 22 × #14, 16 × #12 or 10 × #10; 1 in EMT holds 35 × #14, 26 × #12 or 16 × #10. Open the EMT tab in the chart for every size to 4 in and every conductor to 4/0.
1/2 in PVC Sch 40 holds 11 × #14 or 8 × #12 THHN; 3/4 in holds 21 × #14 or 15 × #12 — one or two fewer than EMT of the same trade size, because the PVC wall is thicker and the bore smaller (0.285 vs 0.304 in² at 1/2 in). Schedule 80 PVC is smaller again. Open the PVC 40 tab.
1/2 in RMC holds 12 × #14 or 9 × #12 THHN; 3/4 in holds 22 × #14 or 16 × #12 — the same counts as EMT at these sizes because the internal areas are nearly identical (0.314 vs 0.304 in²), diverging at 2 in and above where RMC is slightly smaller. IMC sits between them. Open the RMC tab.
Add up the conductor areas, find the limit for that count (40% for three or more), and pick the smallest trade size whose internal area times that limit is at least the total. The Check-a-fill mode reports this as “smallest passing size” every time it runs.
Worked example: two #6 THHN (2 × 0.0507 = 0.1014 in²) in EMT. Two conductors means 31%. 1/2 in EMT allows 0.31 × 0.304 = 0.0942 in² — fails. 3/4 in EMT allows 0.31 × 0.533 = 0.1652 in² — passes. Answer: 3/4 in EMT. Note that adding a third conductor (a ground) would raise the limit to 40%, and 3 × 0.0507 = 0.1521 in² against 0.40 × 0.304 = 0.1216 in² still fails 1/2 in, so 3/4 in stays the answer.
A conduit nipple — a raceway no longer than 24 in installed between boxes, cabinets or enclosures — may be filled to 60% of its internal area regardless of conductor count. This is a note in the code, not the default, so the calculator applies it only when you tick the nipple box. The 0.8 rounding rule still applies to same-size conductors under the 60% limit. A nipple that long also skips the ampacity adjustment for more than three current-carrying conductors, which this page does not calculate.
Conductor area 3 × 0.0133 = 0.0399 in². Three conductors: 40% of 0.304 = 0.1216 in² allowed. Fill = 0.0399 ÷ 0.304 = 13.1% of the conduit, 32.8% of the allowance. Pass. The same conduit would take 9 × #12 before failing.
0.1098 + 0.0422 = 0.1520 in². Five conductors, 40% of 0.533 = 0.2132 in². 28.5% of the conduit, 71.3% of the allowance. Pass. No rounding rule is involved because the sizes are mixed.
3 × 0.3237 + 0.0824 = 1.0535 in². Four conductors, 40% of 3.356 = 1.3424 in². 31.4% of the conduit, 78.5% of the allowance. Pass. Try the same set in 1½ in EMT (0.40 × 2.036 = 0.8144 in²): it fails, so 2 in is the smallest passing size.
Fill limits (53% for one conductor, 31% for two, 40% for three or more) and the conductor and conduit dimensions used here are the industry-standard values from NEC Chapter 9; this page computes results from those dimensions rather than copying any chart. Conduit internal areas cover EMT, IMC, RMC, PVC Schedule 40, PVC Schedule 80, FMC and LFNC-B in trade sizes 1/2 to 4 (LFNC-B to 2); conductor areas cover THHN/THWN/THWN-2, XHHW-2, THW/THW-2 and TW from 14 AWG to 750 kcmil, insulation included. Compact-stranded, RHH/RHW with outer covering, and cable assemblies (MC, NM, SE) are not in the list — use the manufacturer’s overall diameter for those.
It does not calculate ampacity derating for more than three current-carrying conductors, conduit bending or pull tension, jam ratio, or box fill. Fill is a spatial check; sizing the conductors themselves is a separate step (voltage drop, ampacity). Local amendments and the authority having jurisdiction govern.
Three frictions this page fixes, from the top results for these queries: (1) most calculators accept only one conductor size at a time, or one row that must be scrolled through on a phone — here you add as many rows of mixed sizes and insulations as the run has; (2) the chart versions show a count but not the fill percentage, and calculators show a percentage but not the smallest passing size — this page shows the percent of conduit, the percent of allowance, the verdict and the size that passes, in one result; (3) the 0.8 rounding rule is applied silently or not at all — here it is applied, explained, and flagged in every result and every chart cell where it changed the answer. No login, no email, nothing sent anywhere.