format numbers with 6 significant digits

app/views/helpers/money_helper.rb

@@ -10,6 +10,12 @@ def self.format(money)
   end
 
   def self.format_with_precision(amount_cents, currency)
+    amount_cents = if amount_cents < 1
+      BigDecimal("%.6g" % amount_cents.frac)
+    else
+      amount_cents.round(6)
+    end
+
     Utils::MoneyWithPrecision.from_amount(amount_cents, currency).format(
       format: I18n.t('money.format'),
       decimal_mark: I18n.t('money.decimal_mark'),

spec/views/helpers/money_helper_spec.rb

@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe MoneyHelper do
+  subject(:helper) { described_class }
+
+  describe '.format_with_precision' do
+    it 'rounds big decimals to 6 digits' do
+      html = helper.format_with_precision(BigDecimal("123.12345678"), "USD")
+
+      expect(html).to eq('$123.123457')
+    end
+
+    it 'shows six significant digits for values < 1' do
+      html = helper.format_with_precision(BigDecimal("0.000000012345"), "USD")
+
+      expect(html).to eq('$0.000000012345')
+    end
+
+    it 'shows only six significant digits for values < 1' do
+      html = helper.format_with_precision(BigDecimal("0.100000012345"), "USD")
+
+      expect(html).to eq('$0.10')
+    end
+  end
+end