Horsepower → kilowatts
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<!doctype html><html lang="en-US"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width,initial-scale=1"><title>Horsepower → kilowatts</title><style>*{box-sizing:border-box}html{color-scheme:light}body{margin:0;background:var(--bg,#eef1f4);color:var(--ink,#233449);font:16px/1.5 system-ui,-apple-system,Segoe UI,sans-serif}main{max-width:1040px;margin:auto;padding:26px 28px 44px}header{margin-bottom:20px}h1{font-size:clamp(26px,4vw,36px);letter-spacing:-.035em;line-height:1.15;margin:5px 0 10px}h2{font-size:21px;line-height:1.3;margin:0 0 15px}h3{font-size:17px;margin:0 0 9px}p{margin:8px 0 16px}.kicker{font-size:12px;text-transform:uppercase;letter-spacing:.12em;font-weight:750;color:var(--accent,#315b87)}.muted,small{color:#556473}.panel{margin:20px 0;padding:24px;border:1px solid var(--line,#ccd5df);border-radius:10px;background:#fff}.hero{padding:24px;background:var(--hero,#183b5c);color:#fff;border-radius:12px;margin:18px 0}.hero .kicker,.hero 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#9b734d}.comparison{display:grid;grid-template-columns:repeat(3,minmax(0,1fr));gap:20px}.comparison>div{padding:10px 0}.comparison span{display:block;font-size:13px;min-height:39px}.comparison b{display:block;font:clamp(22px,3vw,33px)/1.3 Georgia,serif;color:#754c2d}.comparison small{display:block;margin-top:8px}#current{border-top:2px solid #d7c4b0;padding-top:20px;margin-top:20px}#current .big{font-family:Georgia,serif}@media(max-width:540px){.comparison{grid-template-columns:1fr;gap:0}.comparison>div{display:grid;grid-template-columns:1fr 1fr;gap:6px;border-bottom:1px solid #ddd0c1}.comparison span{min-height:0}.comparison small{grid-column:2;margin-top:0}}</style></head><body style="--bg:#f5f1eb;--accent:#8b613c;--ink:#40362e"><main><header><div class="kicker">One horsepower · three printed-factor results</div><h1>Horsepower → kilowatts</h1><p class="muted">The horsepower definition belongs beside every power result.</p></header><section class="surface horsepower"><div class="comparison"><div><span>Mechanical · 550 ft·lbf/s</span><b>0.7456999</b><small>kW per hp</small></div><div><span>Metric</span><b>0.7354988</b><small>kW per hp</small></div><div><span>Electric</span><b>0.746</b><small>kW per hp</small></div></div></section><form id="form" class="panel" novalidate><div class="row"><label for="type">Horsepower definition<select id="type"><option value="">Choose the source definition</option><option value="mechanical">Mechanical · 550 ft·lbf/s</option><option value="metric">Metric horsepower</option><option value="electric">Electric horsepower</option></select></label><label for="direction">Direction<select id="direction"><option value="forward">hp → kW</option><option value="inverse">kW → hp</option></select></label><label for="quantity">Power quantity · 0 to 1,000,000<input id="quantity" value="2" inputmode="decimal" autocomplete="off"></label></div><p id="origin" class="muted">Manual input · up to six fractional digits</p><div class="actions"><button>Convert power</button><button type="button" id="swap" class="secondary" disabled>Swap direction</button><button type="button" id="reset" class="secondary">Reset</button></div><p id="error" class="error" role="alert"></p><div id="current" hidden aria-live="polite"><div id="answer" class="big"></div><p id="precision"></p></div><p id="swapReason" class="muted"></p></form><section class="panel"><h2>Your power record</h2><pre id="trace"></pre></section><details><summary>Factor precision and limits</summary><p>Mechanical, metric, and electric factors are the NIST printed values 745.6999,735.4988, and 746 W/hp. Extra digits from another source or definition are not added. The type selector starts unselected; a generic hp label does not identify its definition.</p><p>Manual input is a nonnegative dot decimal up to 1,000,000 with at most six fractional digits and leading digits. Blank, negative, comma-formatted, scientific notation and unknown type are rejected. Displays use up to nine fractional digits where needed; tiny positives remain nonzero. Swap uses the unrounded internal ratio, visibly labeled calculated, with a separate range check. Editing returns to manual validation. Reset restores empty input/type and hp→kW.</p><p>This changes units of a supplied power quantity. It does not infer engine displacement, motor input, efficiency, three-phase/circuit requirements or equipment suitability. <a href="https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b9">NIST selected power factors</a></p></details><footer>Local session only · No network or saved progress. Keep maintained source records authoritative.</footer></main><script>const $=id=>document.getElementById(id);const esc=s=>String(s).replace(/[&<>"']/g,c=>({'&':'&','<':'<','>':'>','"':'"',"'":'''}[c]));
function gcd(a,b){return b?gcd(b,a%b):a}function rat(n,d=1n){if(d<=0n||n<0n)throw Error('Nonnegative finite quantity required.');const g=gcd(n,d);return {n:n/g,d:d/g}}function times(a,b){return rat(a.n*b.n,a.d*b.d)}function divide(a,b){if(!b.n)throw Error('Zero factor.');return rat(a.n*b.d,a.d*b.n)}function exactDecimal(s){const [a,b='']=s.split('.');return rat(BigInt(a+b),10n**BigInt(b.length))}function manual(raw,max){const s=raw.trim();if(!/^[0-9]+(?:[.][0-9]{1,6})?$/.test(s)||s.length>30)throw Error('Enter a nonnegative dot-decimal number with up to six places; blank, signs, commas and exponents are invalid.');const r=exactDecimal(s);if(r.n>BigInt(max)*r.d)throw Error('Manual quantity exceeds '+max+'.');return r}function format(r,p=6){const scale=10n**BigInt(p),q=(r.n*scale*2n+r.d)/(2n*r.d),s=q.toString().padStart(p+1,'0'),text=(p?s.slice(0,-p)+'.'+s.slice(-p):s).replace(/([.][0-9]*?)0+$/,'$1').replace(/[.]$/,'');return {text:r.n>0n&&!q?'<'+(1/Number(scale)).toFixed(p):text,approx:r.n*scale%r.d!==0n}}function within(r,max){return r.n<=BigInt(max)*r.d}
const factors={mechanical:'745.6999',metric:'735.4988',electric:'746'};let record=null,computed=null;function convert(r,type,direction){if(!(Object.hasOwn(factors,type)))throw Error('Choose mechanical, metric, or electric horsepower; generic hp is unresolved.');if(!['forward','inverse'].includes(direction))throw Error('Choose a supported direction.');const factor=exactDecimal(factors[type]),thousand=rat(1000n),watts=direction==='forward'?times(r,factor):times(r,thousand),result=direction==='forward'?divide(watts,thousand):divide(watts,factor);return {input:r,type,direction,watts,result}}
function clear(){record=null;$('current').hidden=true;$('trace').textContent='';$('error').textContent='';$('swap').disabled=true;$('swapReason').textContent='A valid conversion is required before Swap.'}function changed(){computed=null;$('origin').textContent='Manual input · up to six fractional digits';clear()}function run(){try{const r=computed||manual($('quantity').value,1000000);if(!within(r,1000000))throw Error('Calculated source quantity exceeds 1,000,000.');record=convert(r,$('type').value,$('direction').value);const f=format(record.result,9),w=format(record.watts,9),unit=record.direction==='forward'?'kW':record.type+' hp';$('answer').textContent=f.text+' '+unit;$('precision').textContent='Horsepower definition: '+record.type+'. '+(f.approx?'Display rounded to nine places; internal ratio remains unrounded.':'Arithmetic retains the selected printed factor precision; not a measurement claim.');$('trace').textContent='Input text: '+$('quantity').value+'\nOrigin: '+(computed?'Calculated, unrounded internal quantity':'Manual supplied quantity')+'\nDirection: '+(record.direction==='forward'?'hp → kW':'kW → hp')+'\nDefinition: '+record.type+'\nPrinted factor: '+factors[record.type]+' W per hp\nWatts: '+w.text+' W'+(w.approx?' (display rounded)':'')+'\nResult: '+f.text+' '+unit+'\nInternal result ratio: '+record.result.n+'/'+record.result.d+'\nSource: '+"https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b9";$('current').hidden=false;$('error').textContent='';$('swap').disabled=!within(record.result,1000000);$('swapReason').textContent=$('swap').disabled?'Swap unavailable: new source quantity would exceed 1,000,000.':'Swap reverses direction with this stored unrounded result and the same definition.'}catch(e){clear();$('error').textContent=e.message}}
$('form').addEventListener('submit',e=>{e.preventDefault();run()});$('quantity').addEventListener('input',changed);['type','direction'].forEach(x=>$(x).addEventListener('change',changed));$('swap').onclick=()=>{if(!record||!within(record.result,1000000))return;const old=record;computed=old.result;$('quantity').value=format(computed,12).text;$('direction').value=old.direction==='forward'?'inverse':'forward';$('origin').textContent='Calculated input · stored unrounded ratio '+computed.n+'/'+computed.d;run()};$('reset').onclick=()=>{computed=null;$('quantity').value='';$('type').value='';$('direction').value='forward';clear();$('swapReason').textContent='';$('origin').textContent='Manual input · up to six fractional digits'};clear();</script>
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