:root{
  --bg:#0f172a;
  --bg-soft:#1e293b;
  --card:#ffffff;
  --ink:#0f172a;
  --ink-soft:#475569;
  --line:#e2e8f0;
  --brand:#f97316;
  --brand-dk:#ea580c;
  --accent:#0ea5e9;
  --good:#16a34a;
  --shadow:0 10px 30px rgba(2,6,23,.08);
  --radius:14px;
  --ff:'Segoe UI',system-ui,-apple-system,Roboto,Helvetica,Arial,sans-serif;
}
*{box-sizing:border-box}
html{scroll-behavior:smooth}
body{
  margin:0;font-family:var(--ff);color:var(--ink);
  background:#f1f5f9;line-height:1.6;
}
.wrap{width:min(1100px,92%);margin-inline:auto}
a{color:var(--brand-dk);text-decoration:none}
h1,h2{line-height:1.2}

/* Header */
.site-header{position:sticky;top:0;z-index:30;background:rgba(15,23,42,.92);
  backdrop-filter:blur(8px);border-bottom:1px solid rgba(255,255,255,.06)}
.header-inner{display:flex;align-items:center;justify-content:space-between;height:62px}
.brand{display:flex;align-items:center;gap:.5rem;color:#fff;font-size:1.15rem}
.brand-mark{display:grid;place-items:center;width:34px;height:34px;border-radius:9px;
  background:linear-gradient(135deg,var(--brand),var(--brand-dk));color:#fff;font-size:1.1rem}
.brand-text strong{color:var(--brand)}
.top-nav{display:flex;gap:1.3rem}
.top-nav a{color:#cbd5e1;font-size:.93rem}
.top-nav a:hover{color:#fff}
.lang-dropdown{position:relative;margin-left:1rem}
.lang-dropdown summary{list-style:none;cursor:pointer;color:#cbd5e1;font-size:.85rem;font-weight:600;
  padding:.35rem .7rem;border:1px solid rgba(255,255,255,.18);border-radius:8px;
  display:flex;align-items:center;gap:.4rem;white-space:nowrap}
.lang-dropdown summary::-webkit-details-marker{display:none}
.lang-dropdown summary:hover{color:#fff;border-color:rgba(255,255,255,.45)}
.lang-dropdown .caret{transition:transform .2s;font-size:.7rem}
.lang-dropdown[open] .caret{transform:rotate(180deg)}
.lang-menu{position:absolute;right:0;top:calc(100% + .45rem);background:#fff;border-radius:10px;
  box-shadow:0 14px 34px rgba(2,6,23,.28);padding:.4rem;display:grid;
  grid-template-columns:1fr 1fr;gap:.1rem;min-width:300px;max-height:70vh;overflow:auto;z-index:60}
.lang-menu .lang{color:var(--ink-soft);font-size:.86rem;padding:.45rem .65rem;border-radius:7px;white-space:nowrap}
.lang-menu .lang:hover{background:#f1f5f9;color:var(--ink)}
.lang-menu .lang.active{background:var(--brand);color:#fff;font-weight:600}
@media(max-width:560px){.lang-menu{grid-template-columns:1fr;min-width:180px}}

/* Hero */
.hero{background:linear-gradient(160deg,#0f172a,#1e293b 60%,#0b3a52);
  color:#fff;padding:3rem 0 3.4rem;text-align:center}
.hero h1{font-size:clamp(2rem,5vw,3rem);margin:0 0 .6rem}
.hero-sub{max-width:640px;margin:0 auto 1.4rem;color:#cbd5e1;font-size:1.05rem}
.hero-sub strong{color:#fff}
.hero-badges{list-style:none;display:flex;flex-wrap:wrap;gap:.6rem;justify-content:center;padding:0;margin:0}
.hero-badges li{background:rgba(255,255,255,.08);border:1px solid rgba(255,255,255,.14);
  padding:.35rem .8rem;border-radius:999px;font-size:.85rem;color:#e2e8f0}

/* Calculator shell */
.calc-shell{display:grid;grid-template-columns:240px 1fr;gap:1.4rem;
  margin:-2rem auto 2.5rem;position:relative;z-index:5}
.calc-tabs{display:flex;flex-direction:column;gap:.4rem;background:var(--card);
  padding:.7rem;border-radius:var(--radius);box-shadow:var(--shadow);height:fit-content}
.tab-group{font-size:.7rem;text-transform:uppercase;letter-spacing:.08em;
  color:#94a3b8;font-weight:700;padding:.7rem .85rem .25rem}
.tab{text-align:left;border:none;background:transparent;padding:.55rem .85rem;
  border-radius:10px;font-size:.9rem;color:var(--ink-soft);cursor:pointer;
  font-family:inherit;transition:.15s;font-weight:500;width:100%}
.tab:hover{background:#f1f5f9;color:var(--ink)}
.tab.active{background:linear-gradient(135deg,var(--brand),var(--brand-dk));color:#fff;
  box-shadow:0 6px 14px rgba(249,115,22,.35)}

.calc-panel{background:var(--card);padding:1.6rem;border-radius:var(--radius);box-shadow:var(--shadow)}
.calc-panel h2{margin:0 0 .25rem;font-size:1.35rem}
.calc-desc{margin:0 0 1.2rem;color:var(--ink-soft);font-size:.95rem}

.controls,.inputs,.qty-row{display:grid;gap:1rem}
.controls{grid-template-columns:1fr 1fr;margin-bottom:1rem}
.inputs{grid-template-columns:repeat(auto-fit,minmax(150px,1fr));margin-bottom:1rem}
.qty-row{grid-template-columns:200px;margin-bottom:1.4rem}
.field{display:flex;flex-direction:column;gap:.3rem}
.field label{font-size:.82rem;font-weight:600;color:var(--ink-soft)}
.field input,.field select{padding:.6rem .7rem;border:1.5px solid var(--line);
  border-radius:10px;font-size:1rem;font-family:inherit;background:#fff;transition:.15s}
.field input:focus,.field select:focus{outline:none;border-color:var(--accent);
  box-shadow:0 0 0 3px rgba(14,165,233,.15)}

/* Result */
.result{background:linear-gradient(135deg,#0f172a,#1e293b);color:#fff;
  border-radius:var(--radius);padding:1.4rem 1.5rem}
.result-main{display:flex;align-items:baseline;gap:.5rem;margin-bottom:.1rem}
.result-val{font-size:2.6rem;font-weight:700;color:var(--brand);letter-spacing:-1px}
.result-unit{color:#cbd5e1;font-size:1rem}
.result-label{color:#94a3b8;font-size:.8rem;text-transform:uppercase;
  letter-spacing:.05em;margin-bottom:1rem}
.result-grid{display:grid;grid-template-columns:repeat(auto-fit,minmax(120px,1fr));gap:.8rem;
  border-top:1px solid rgba(255,255,255,.12);padding-top:1rem}
.result-grid>div{display:flex;flex-direction:column}
.result-grid span{font-size:1.25rem;font-weight:600}
.result-grid small{color:#94a3b8;font-size:.74rem;text-transform:uppercase;letter-spacing:.04em}
.result-formula{margin:1rem 0 0;font-size:.85rem;color:#94a3b8;font-family:ui-monospace,Consolas,monospace}

/* Related */
.related{margin-bottom:3rem}
.related h2{font-size:1.45rem;margin-bottom:1rem}
.related-grid{display:grid;grid-template-columns:repeat(auto-fill,minmax(220px,1fr));gap:1rem}
.rel-card{text-align:left;background:var(--card);border:1.5px solid var(--line);
  border-radius:12px;padding:1rem 1.1rem;cursor:pointer;font-family:inherit;
  transition:.18s;display:flex;flex-direction:column;gap:.2rem}
.rel-card b{font-size:1.02rem;color:var(--ink)}
.rel-card span{font-size:.85rem;color:var(--ink-soft)}
.rel-card:hover{border-color:var(--brand);transform:translateY(-3px);box-shadow:var(--shadow)}

/* Content blocks */
.content-block{background:var(--card);border-radius:var(--radius);padding:1.8rem 2rem;
  margin-bottom:1.6rem;box-shadow:var(--shadow)}
.content-block h2{font-size:1.5rem;margin:0 0 1rem}
.formula-callout{background:#0f172a;color:var(--brand);font-weight:700;font-size:1.2rem;
  text-align:center;padding:.9rem;border-radius:10px;font-family:ui-monospace,Consolas,monospace}
.formula-table{overflow-x:auto}
.formula-table table{width:100%;border-collapse:collapse;margin-top:.5rem}
.formula-table th,.formula-table td{text-align:left;padding:.65rem .8rem;border-bottom:1px solid var(--line)}
.formula-table th{background:#f8fafc;font-size:.85rem;text-transform:uppercase;letter-spacing:.03em;color:var(--ink-soft)}
.formula-table td small{color:var(--ink-soft)}
.chart-2col{display:grid;grid-template-columns:1fr 1fr;gap:1.5rem}
.chart-2col h3{margin:.2rem 0 .5rem;font-size:1.05rem}
@media(max-width:700px){.chart-2col{grid-template-columns:1fr}}

/* breadcrumbs */
.crumbs{font-size:.82rem;color:var(--ink-soft);padding:.7rem 0 .1rem}
.crumbs a{color:var(--brand-dk);text-decoration:none}
.crumbs a:hover{text-decoration:underline}
.crumbs i{color:#94a3b8;font-style:normal;margin:0 .15rem}
.crumbs span{color:var(--ink)}
.silo-all{margin-top:1rem}
.silo-all a{font-weight:600;color:var(--brand-dk)}
.dir-cat h3 a{color:var(--brand-dk);text-decoration:none}
.dir-cat h3 a:hover{text-decoration:underline}

/* how-to + directory */
.howto{margin:.3rem 0 1.2rem;padding-left:1.2rem}
.howto li{margin-bottom:.45rem;color:var(--ink-soft)}
.content-block h3{font-size:1.1rem;margin:1.2rem 0 .5rem}
.dir-cat{margin-bottom:1.6rem}
.dir-cat h3{font-size:1.05rem;color:var(--brand-dk);margin:.2rem 0 .7rem;
  border-bottom:2px solid var(--line);padding-bottom:.3rem}

/* FAQ */
details{border:1px solid var(--line);border-radius:10px;padding:.4rem 1rem;margin-bottom:.7rem}
details[open]{background:#f8fafc}
summary{cursor:pointer;font-weight:600;padding:.5rem 0}
details p{margin:.2rem 0 .8rem;color:var(--ink-soft)}

/* Footer */
.site-footer{background:#0f172a;color:#cbd5e1;padding:2rem 0;text-align:center;margin-top:2rem}
.site-footer p{margin:.2rem 0}
.muted{color:#64748b;font-size:.85rem}

/* Responsive */
@media(max-width:820px){
  .calc-shell{grid-template-columns:1fr}
  .calc-tabs{flex-direction:row;flex-wrap:wrap;margin-top:1rem}
  .tab{flex:1 1 auto}
  .top-nav{display:none}
  .result-grid{grid-template-columns:1fr 1fr}
}