diff --git a/src/app.css b/src/app.css
new file mode 100644
index 0000000..516c499
--- /dev/null
+++ b/src/app.css
@@ -0,0 +1,115 @@
+/* app.css — global styles for humor-units */
+
+/* CSS custom properties (set dynamically via inline style from ConverterCard) */
+:root {
+  --min-col-width: 160px;
+  --grid-gap: 1rem;
+}
+
+/* ── Results grid ──────────────────────────────────────────────────────────── */
+.results-grid {
+  display: grid;
+  grid-template-columns: repeat(auto-fill, minmax(var(--min-col-width), 1fr));
+  gap: var(--grid-gap);
+  margin-top: 1.5rem;
+}
+
+/* ── Result tile ───────────────────────────────────────────────────────────── */
+.result-tile {
+  cursor: pointer;
+  padding: 0.75rem 1rem;
+  border: 1px solid var(--pico-muted-border-color, #ccc);
+  border-radius: var(--pico-border-radius, 0.25rem);
+  display: flex;
+  flex-direction: column;
+  gap: 0.2rem;
+  transition: background-color 0.15s ease, border-color 0.15s ease;
+  user-select: none;
+}
+
+.result-tile:hover {
+  background-color: var(--pico-secondary-background, rgba(0, 0, 0, 0.04));
+  border-color: var(--pico-primary, #1095c1);
+}
+
+.result-tile.highlighted {
+  border-color: var(--pico-primary, #1095c1);
+  background-color: var(--pico-primary-background, rgba(16, 149, 193, 0.08));
+}
+
+.result-value {
+  font-size: 1rem;
+  font-weight: 600;
+  font-variant-numeric: tabular-nums;
+  word-break: break-all;
+}
+
+.result-label {
+  font-size: 0.85rem;
+  color: var(--pico-muted-color, #666);
+}
+
+.result-symbol {
+  font-size: 0.75rem;
+  color: var(--pico-muted-color, #666);
+  font-style: italic;
+}
+
+/* ── Highlighted result (above grid) ──────────────────────────────────────── */
+.highlighted-result {
+  text-align: center;
+  padding: 2rem 1rem;
+  margin-bottom: 1.5rem;
+  border-radius: var(--pico-border-radius, 0.25rem);
+  background-color: var(--pico-primary-background, rgba(16, 149, 193, 0.08));
+  border: 2px solid var(--pico-primary, #1095c1);
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  gap: 0.4rem;
+}
+
+.highlighted-value {
+  font-size: 2.5rem;
+  font-weight: 700;
+  font-variant-numeric: tabular-nums;
+  line-height: 1.1;
+  word-break: break-all;
+}
+
+.highlighted-label {
+  font-size: 1.25rem;
+  color: var(--pico-color, inherit);
+}
+
+.highlighted-symbol {
+  font-size: 0.9rem;
+  color: var(--pico-muted-color, #666);
+  font-style: italic;
+}
+
+/* ── Converter controls ────────────────────────────────────────────────────── */
+.converter-controls {
+  display: flex;
+  gap: 1rem;
+  align-items: center;
+  flex-wrap: wrap;
+}
+
+.converter-controls input[type='number'] {
+  flex: 1 1 200px;
+  min-width: 120px;
+}
+
+.converter-controls select {
+  flex: 0 1 auto;
+  min-width: 160px;
+}
+
+/* ── Hotkey hint ───────────────────────────────────────────────────────────── */
+.hotkey-hint {
+  margin-top: 1rem;
+  margin-bottom: 0;
+  color: var(--pico-muted-color, #666);
+  text-align: center;
+}
diff --git a/src/components/ConversionResult.svelte b/src/components/ConversionResult.svelte
new file mode 100644
index 0000000..f73683a
--- /dev/null
+++ b/src/components/ConversionResult.svelte
@@ -0,0 +1,52 @@
+<script lang="ts">
+  import type { Big } from 'big.js';
+  import type { Unit } from '$lib/types';
+  import { formatBig } from '$lib/format';
+
+  /**
+   * ConversionResult — displays one conversion result tile.
+   * Emits onclick (highlight) and onshiftclick (make input unit) events.
+   */
+
+  let {
+    value,
+    unit,
+    highlighted,
+    onclick,
+    onshiftclick
+  }: {
+    value: Big;
+    unit: Unit;
+    highlighted: boolean;
+    onclick: () => void;
+    onshiftclick: () => void;
+  } = $props();
+
+  // Compute once per reactive update; shared between aria-label and display span.
+  let formattedValue = $derived(formatBig(value));
+
+  function handleClick(e: MouseEvent) {
+    if (e.shiftKey) {
+      onshiftclick();
+    } else {
+      onclick();
+    }
+  }
+</script>
+
+<!-- svelte-ignore a11y_no_static_element_interactions -->
+<!-- svelte-ignore a11y_click_events_have_key_events -->
+<div
+  class="result-tile"
+  class:highlighted
+  onclick={handleClick}
+  role="button"
+  tabindex="0"
+  onkeydown={(e) => { if (e.key === 'Enter') onclick(); }}
+  aria-label="{formattedValue} {unit.labelPlural}"
+  aria-pressed={highlighted}
+>
+  <span class="result-value">{formattedValue}</span>
+  <span class="result-label">{unit.labelPlural}</span>
+  <span class="result-symbol">{unit.symbol}</span>
+</div>
diff --git a/src/components/ConverterCard.svelte b/src/components/ConverterCard.svelte
new file mode 100644
index 0000000..64608d5
--- /dev/null
+++ b/src/components/ConverterCard.svelte
@@ -0,0 +1,59 @@
+<script lang="ts">
+  import type { Unit, ResultItem } from '$lib/types';
+  import { MIN_COLUMN_WIDTH, GRID_GAP } from '$lib/config';
+  import UnitInput from './UnitInput.svelte';
+  import UnitDropdown from './UnitDropdown.svelte';
+  import ConversionResult from './ConversionResult.svelte';
+
+  /**
+   * ConverterCard — groups UnitInput + UnitDropdown + results grid + hotkey hint.
+   * Emits events upward; does NOT contain NL input (Phase 3).
+   */
+
+  let {
+    units,
+    inputValue,
+    fromUnitId,
+    highlightedUnitId,
+    results,
+    oninputchange,
+    onunitchange,
+    onhighlight,
+    onsetinput
+  }: {
+    units: Unit[];
+    inputValue: number;
+    fromUnitId: string;
+    highlightedUnitId: string | null;
+    results: ResultItem[];
+    oninputchange: (v: number) => void;
+    onunitchange: (id: string) => void;
+    onhighlight: (id: string) => void;
+    onsetinput: (id: string) => void;
+  } = $props();
+
+  const gridStyle = `--min-col-width: ${MIN_COLUMN_WIDTH}; --grid-gap: ${GRID_GAP};`;
+</script>
+
+<article>
+  <div class="converter-controls">
+    <UnitInput value={inputValue} onchange={oninputchange} />
+    <UnitDropdown {units} value={fromUnitId} onchange={onunitchange} />
+  </div>
+
+  <div class="results-grid" style={gridStyle}>
+    {#each results as item (item.unit.id)}
+      <ConversionResult
+        value={item.convertedValue}
+        unit={item.unit}
+        highlighted={item.unit.id === highlightedUnitId}
+        onclick={() => onhighlight(item.unit.id)}
+        onshiftclick={() => onsetinput(item.unit.id)}
+      />
+    {/each}
+  </div>
+
+  <p class="hotkey-hint">
+    <small>Click a result to highlight it · Shift+click to use it as input</small>
+  </p>
+</article>
diff --git a/src/components/UnitDropdown.svelte b/src/components/UnitDropdown.svelte
new file mode 100644
index 0000000..5405303
--- /dev/null
+++ b/src/components/UnitDropdown.svelte
@@ -0,0 +1,29 @@
+<script lang="ts">
+  import type { Unit } from '$lib/types';
+
+  /**
+   * UnitDropdown — select element populated from a Unit[] prop.
+   * Emits onchange with the selected unit ID string.
+   */
+
+  let {
+    units,
+    value,
+    onchange
+  }: {
+    units: Unit[];
+    value: string;
+    onchange: (id: string) => void;
+  } = $props();
+
+  function handleChange(e: Event) {
+    const select = e.currentTarget as HTMLSelectElement;
+    onchange(select.value);
+  }
+</script>
+
+<select value={value} onchange={handleChange} aria-label="From unit">
+  {#each units as unit (unit.id)}
+    <option value={unit.id}>{unit.label} ({unit.symbol})</option>
+  {/each}
+</select>
diff --git a/src/components/UnitInput.svelte b/src/components/UnitInput.svelte
new file mode 100644
index 0000000..6632c85
--- /dev/null
+++ b/src/components/UnitInput.svelte
@@ -0,0 +1,30 @@
+<script lang="ts">
+  /**
+   * UnitInput — controlled numeric input.
+   * Phase 2: number input only. NL field added in Phase 3.
+   */
+
+  let {
+    value,
+    onchange
+  }: {
+    value: number;
+    onchange: (v: number) => void;
+  } = $props();
+
+  function handleInput(e: Event) {
+    const input = e.currentTarget as HTMLInputElement;
+    const parsed = parseFloat(input.value);
+    if (!isNaN(parsed)) {
+      onchange(parsed);
+    }
+  }
+</script>
+
+<input
+  type="number"
+  step="any"
+  value={value}
+  oninput={handleInput}
+  aria-label="Input value"
+/>
diff --git a/src/lib/format.ts b/src/lib/format.ts
new file mode 100644
index 0000000..c045a42
--- /dev/null
+++ b/src/lib/format.ts
@@ -0,0 +1,16 @@
+import type { Big } from 'big.js';
+import { FRACTIONAL_DIGITS, INTEGER_DIGITS } from './config';
+
+/**
+ * Format a Big value for display.
+ * Uses FRACTIONAL_DIGITS decimal places (fixed-point).
+ * If INTEGER_DIGITS is set, zero-pads the integer portion to that width.
+ */
+export function formatBig(v: Big): string {
+	const fixed = v.toFixed(FRACTIONAL_DIGITS);
+	if (INTEGER_DIGITS === undefined) return fixed;
+
+	const [intPart, fracPart] = fixed.split('.');
+	const paddedInt = intPart.padStart(INTEGER_DIGITS, '0');
+	return fracPart !== undefined ? `${paddedInt}.${fracPart}` : paddedInt;
+}
diff --git a/src/lib/types.ts b/src/lib/types.ts
index 7ea326f..6624ac0 100644
--- a/src/lib/types.ts
+++ b/src/lib/types.ts
@@ -32,3 +32,9 @@ export interface UnitGroup {
   label: string;
   units: Unit[];
 }
+
+/** One computed conversion result — unit plus its converted Big value. */
+export interface ResultItem {
+  unit: Unit;
+  convertedValue: import('big.js').Big;
+}
diff --git a/src/routes/+layout.svelte b/src/routes/+layout.svelte
index cd92650..cb68cbe 100644
--- a/src/routes/+layout.svelte
+++ b/src/routes/+layout.svelte
@@ -1,5 +1,6 @@
 <script lang="ts">
   import '@picocss/pico/css/pico.min.css';
+  import '../app.css';
 
   let { children } = $props();
 </script>
diff --git a/src/routes/+page.svelte b/src/routes/+page.svelte
index 3aab346..2ca28a3 100644
--- a/src/routes/+page.svelte
+++ b/src/routes/+page.svelte
@@ -1 +1,122 @@
-<h1>Humor Units</h1>
+<script lang="ts">
+  import { browser } from '$app/environment';
+  import { goto } from '$app/navigation';
+  import { page } from '$app/state';
+  import { florpUnits } from '$lib/units/definitions';
+  import { convertById } from '$lib/convert';
+  import { FRACTIONAL_DIGITS } from '$lib/config';
+  import { formatBig } from '$lib/format';
+  import type { Unit, ResultItem } from '$lib/types';
+  import ConverterCard from '../components/ConverterCard.svelte';
+
+  /**
+   * Converter page — owns all reactive state.
+   * Phase 2: live conversion, URL state, highlight interaction.
+   * Phase 3 will add NL input.
+   */
+
+  const units: Unit[] = florpUnits;
+
+  // ── Reactive state ──────────────────────────────────────────────────────────
+  let inputValue = $state<number>(1);
+  let fromUnitId = $state<string>(units[0].id);
+  let highlightedUnitId = $state<string | null>(null);
+
+  // ── Initialize from URL params on first client-side load ───────────────────
+  if (browser) {
+    const params = page.url.searchParams;
+    const pValue = params.get('value');
+    const pFrom = params.get('from');
+    const pHighlight = params.get('highlight');
+
+    if (pValue !== null) {
+      const n = parseFloat(pValue);
+      if (!isNaN(n)) inputValue = n;
+    }
+    if (pFrom !== null && units.some((u) => u.id === pFrom)) {
+      fromUnitId = pFrom;
+    }
+    if (pHighlight !== null && units.some((u) => u.id === pHighlight)) {
+      highlightedUnitId = pHighlight;
+    }
+  }
+
+  // ── Derived: results for all units except fromUnit ─────────────────────────
+  let results = $derived<ResultItem[]>(
+    units
+      .filter((u) => u.id !== fromUnitId)
+      .map((u) => ({
+        unit: u,
+        convertedValue: convertById(inputValue, fromUnitId, u.id, units)
+      }))
+  );
+
+  // ── Derived: highlighted result (unit + value) sourced from results array ──
+  // The highlighted unit is always a non-from unit, so it's always in results.
+  let highlightedResult = $derived(
+    highlightedUnitId !== null
+      ? (results.find((r) => r.unit.id === highlightedUnitId) ?? null)
+      : null
+  );
+
+  // ── URL sync: update URL only when serialized params actually change ────────
+  $effect(() => {
+    if (!browser) return;
+    const params = new URLSearchParams();
+    params.set('value', String(inputValue));
+    params.set('from', fromUnitId);
+    if (highlightedUnitId !== null) {
+      params.set('highlight', highlightedUnitId);
+    }
+    const next = params.toString();
+    if (next !== window.location.search.slice(1)) {
+      goto(`?${next}`, { replaceState: true, noScroll: true });
+    }
+  });
+
+  // ── Event handlers ──────────────────────────────────────────────────────────
+  function handleUnitChange(id: string) {
+    fromUnitId = id;
+    // Clear highlight if the selected unit becomes the new from-unit.
+    if (highlightedUnitId === id) {
+      highlightedUnitId = null;
+    }
+  }
+
+  function handleHighlight(id: string) {
+    highlightedUnitId = id === highlightedUnitId ? null : id;
+  }
+
+  function handleSetInput(id: string) {
+    // Shift+click: promote result unit to from-unit.
+    // Re-express the current value in terms of the new from-unit.
+    const convertedValue = convertById(inputValue, fromUnitId, id, units);
+    inputValue = parseFloat(convertedValue.toFixed(FRACTIONAL_DIGITS));
+    fromUnitId = id;
+    highlightedUnitId = null;
+  }
+</script>
+
+<svelte:head>
+  <title>Humor Units — Converter</title>
+</svelte:head>
+
+{#if highlightedResult !== null}
+  <div class="highlighted-result">
+    <span class="highlighted-value">{formatBig(highlightedResult.convertedValue)}</span>
+    <span class="highlighted-label">{highlightedResult.unit.labelPlural}</span>
+    <span class="highlighted-symbol">{highlightedResult.unit.symbol}</span>
+  </div>
+{/if}
+
+<ConverterCard
+  {units}
+  {inputValue}
+  {fromUnitId}
+  {highlightedUnitId}
+  {results}
+  oninputchange={(v) => (inputValue = v)}
+  onunitchange={handleUnitChange}
+  onhighlight={handleHighlight}
+  onsetinput={handleSetInput}
+/>