401k contribution percentage calculator

Some 401k administrators lack an option to set contributions to level that
achieves the maximum deferrable contribution. For 2017, that amount is $18,000.
This little script will help you calculate what you percentage you need to use
when that's the only thing you can set.

Why the multiple percentage ranges? Some 401k administrators don't let you set
a high precision percentage. These lines will tell you how much you're losing
because of that imprecision.

TL;DR Tell your 401k administrator to get with the program and do deferral
maximization.

#!/usr/bin/env ruby
MAX_DEFERRABLE = ENV['MAX_DEFERRABLE'] || 18_000.00

if ARGV.size <= 1
  me = File.basename($PROGRAM_NAME, File.extname($PROGRAM_NAME))
  STDERR.puts "Usage: #{me} <number of periods remaining> <pretax pay per period> [amount already contributed]"
  STDERR.puts 'Example, for the beginning of the year for someone who makes $60,000/yr with 24 pay periods:'
  STDERR.puts "\t#{me} 24 2500"
  STDERR.puts "Set MAX_DEFERRABLE envvar if it's not #{MAX_DEFERRABLE}."
  exit 1
end

PERIODS_REMAINING = ARGV[0].to_f
PAY_PER_PERIOD = ARGV[1].to_f

ALREADY_CONTRIBUTED = if ARGV.size == 3
                        ARGV[2].to_f
                      else
                        0.0
                      end

remaining = MAX_DEFERRABLE - ALREADY_CONTRIBUTED
per_period = remaining / PERIODS_REMAINING

puts <<~EH
#{MAX_DEFERRABLE} max deferrable.
#{ALREADY_CONTRIBUTED} already contributed.
#{remaining} remaining to be contributed.
#{PERIODS_REMAINING} periods remaining.
#{per_period} contribution per period to reach #{remaining}.
#{PAY_PER_PERIOD} pay each period.
EH

contribution = per_period / PAY_PER_PERIOD
(0...5).each do |places|
  percent = (contribution * 100.0).floor(places)
  year_total = ((percent / 100.0) * PAY_PER_PERIOD * PERIODS_REMAINING).round(2)
  missed_out = (remaining - year_total).round(2)
  puts "#{percent}% is recommended contribution at #{places} decimal places, totaling #{year_total} for the year, missing out on #{missed_out} because of safe decimal truncation."
end