Volume, with its unit system

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<section class="result"><span class="eyebrow">Conversion result</span><div id="answer" class="big">≈ 7.570824 L</div><p id="equation">2 U.S. liquid gallons × 3.785412 L/gallon</p></section>
<form class="panel below" id="form" novalidate><div class="volumeFields"><div><label for="quantity">Supplied volume</label><input id="quantity" value="2" inputmode="decimal"><div class="caption" id="origin">Manually supplied input</div></div><div><label for="from">From</label><select id="from"></select></div><div><label for="to">To</label><select id="to"></select></div></div><div class="buttons"><button type="submit" class="primary">Calculate</button><button type="button" id="swap">Swap using unrounded result</button><button type="button" id="reset">Reset</button></div><p class="error" id="feedback" role="status"></p></form>
<section class="gallons below"><div><span class="eyebrow">2 U.S. liquid gallons</span><h2>7.570824 L</h2></div><div><span class="eyebrow">2 Imperial gallons</span><h2>9.09218 L</h2></div><p>Difference: 1.521356 L from the declared unit definitions.</p></section>
<section class="panel below"><h2>Factor and input record</h2><pre class="record" id="record"></pre><p class="muted">NIST printed factors into L: mL 0.001; L 1; m³ 1000; U.S. liquid gallon 3.785412; Imperial gallon 4.54609. The U.S. factor is rounded as printed. Displays use at most six decimals, with scientific notation for tiny positive values.</p><p class="muted">Manual values: 0–1,000,000, up to six decimal places. Generic “gal” needs clarification. Cups, ounces, dry units and mass are unsupported. This converts supplied volume, not dimensions, contents or certified capacity.</p><a href="https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b9">NIST volume factors</a></section><footer class="muted below">Local working copy · Changes are not saved after this page closes.</footer></main><script data-app>
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const units={ml:{name:'Milliliter (mL)',short:'mL',factor:.001},l:{name:'Liter (L)',short:'L',factor:1},m3:{name:'Cubic meter (m³)',short:'m³',factor:1000},us:{name:'U.S. liquid gallon',short:'U.S. liquid gallons',factor:3.785412},imp:{name:'Imperial gallon',short:'Imperial gallons',factor:4.54609}};let last=null,internal=null;
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