.btuCalc{max-width:980px;margin:20px auto;padding:18px}
.btuCalc h3{margin:0 0 14px}
.btuCalc__grid{display:grid;gap:14px;grid-template-columns:repeat(2,minmax(0,1fr))}
.btuCalc__card{padding:14px;border:1px solid rgba(0,0,0,.08);border-radius:12px}
.btuCalc__card h4{margin:0 0 10px}
.btuCalc label{display:block;font-size:14px;margin:10px 0}
.btuCalc select,.btuCalc input[type=number]{width:100%;padding:10px;border:1px solid rgba(0,0,0,.18);border-radius:10px}
.btuCalc__units{display:flex;gap:14px;margin:8px 0 12px;flex-wrap:wrap}
.btuCalc__units label{display:flex;align-items:center;gap:8px;margin:0}
.btuCalc__dims{display:grid;gap:10px;grid-template-columns:repeat(3,minmax(0,1fr))}
.btuCalc__btn{margin-top:10px;padding:10px 14px;border-radius:12px;border:1px solid rgba(0,0,0,.18);background:#fff;cursor:pointer}
.btuCalc__btn:hover{filter:brightness(.98)}
.btuCalc__err{color:#b00020;margin:10px 0 0}
.btuCalc__result{margin-top:14px;padding:14px;border:1px solid rgba(0,0,0,.08);border-radius:12px}
.btuCalc__resultGrid{display:grid;gap:12px;grid-template-columns:repeat(2,minmax(0,1fr));margin-top:12px}
.btuCalc__stat{padding:12px;border:1px solid rgba(0,0,0,.08);border-radius:12px}
.btuCalc__statLabel{font-size:13px;opacity:.75}
.btuCalc__statValue{font-size:20px;font-weight:700;margin-top:6px}
@media (max-width:820px){
  .btuCalc__grid,.btuCalc__resultGrid{grid-template-columns:1fr}
  .btuCalc__dims{grid-template-columns:1fr}
}