diff --git a/crates/calc/src/recalc.rs b/crates/calc/src/recalc.rs
index aae212dd3..7c3a6f361 100644
--- a/crates/calc/src/recalc.rs
+++ b/crates/calc/src/recalc.rs
@@ -22,6 +22,13 @@ use crate::spill_index::{SpillIndex, indexing_pays};
 /// Changed cells (in one `cells_changed`) from which a sheet's spill areas may be indexed.
 const INDEX_SPILLS_FROM: usize = 16;
 
+/// Whether a sheet's spill areas are indexed for a batch of `batch` changed cells of which
+/// `on_sheet` are on that sheet: the batch must be a paste, and registering the areas must cost
+/// less than walking them once for each of the sheet's own changed cells.
+fn index_spills<'a>(batch: usize, on_sheet: usize, areas: impl ExactSizeIterator<Item = &'a RangeRef>) -> bool {
+    batch > INDEX_SPILLS_FROM && areas.len() > 0 && indexing_pays(areas, on_sheet)
+}
+
 mod graph;
 mod pass_host;
 
@@ -250,20 +257,22 @@ impl Calc {
         // walked for each one, when registering them costs less than walking them would.
         let mut areas = SpillIndex::default();
         let mut strategy: HashMap<usize, bool> = HashMap::default();
+        let mut per_sheet: HashMap<usize, usize> = HashMap::default();
+        for k in changed {
+            *per_sheet.entry(k.0).or_insert(0) += 1;
+        }
         let mut at = Vec::new();
         for &k in changed {
             if let Some(sh) = wb.sheet(k.0) {
-                let indexed = changed.len() > INDEX_SPILLS_FROM
-                    && !sh.spill_ranges.is_empty()
-                    && *strategy.entry(k.0).or_insert_with(|| {
-                        let indexed = indexing_pays(sh.spill_ranges.values(), changed.len());
-                        if indexed {
-                            for (anchor, r) in &sh.spill_ranges {
-                                areas.insert_rect((k.0, *anchor), *r, false);
-                            }
+                let indexed = *strategy.entry(k.0).or_insert_with(|| {
+                    let indexed = index_spills(changed.len(), per_sheet[&k.0], sh.spill_ranges.values());
+                    if indexed {
+                        for (anchor, r) in &sh.spill_ranges {
+                            areas.insert_rect((k.0, *anchor), *r, false);
                         }
-                        indexed
-                    });
+                    }
+                    indexed
+                });
                 if indexed {
                     // Anchor order, as walking `spill_ranges` gives.
                     areas.meeting(k.0, RangeRef::new(k.1, k.1), &mut at);
@@ -556,7 +565,7 @@ pub(crate) mod tests {
     use gridcraft_core::{CellRef, RangeRef, Value};
     use gridcraft_model::{Cell, Formula, Workbook};
 
-    use super::{Calc, HashMap, Host, Intervals, Key, PassHost};
+    use super::{Calc, HashMap, Host, Intervals, Key, PassHost, index_spills};
 
     /// xorshift64 for repeatable pseudo-random cases.
     fn rng(state: &mut u64) -> u64 {
@@ -573,6 +582,19 @@ pub(crate) mod tests {
         v
     }
 
+    /// One changed cell on a sheet with many small spills does not index them just because the
+    /// batch (mostly on another sheet) is large; the sheet's own share of the batch decides.
+    #[test]
+    fn the_spill_index_is_chosen_from_the_sheets_own_changed_cells() {
+        let small: Vec<RangeRef> = (0..1_000).map(|i| RangeRef::new(CellRef::new(i * 3, 0), CellRef::new(i * 3 + 2, 0))).collect();
+        // Same areas, same batch of 5000: indexed when 5000 of them are on the sheet, walked when 1 is.
+        assert!(index_spills(5_000, 5_000, small.iter()));
+        assert!(!index_spills(5_000, 1, small.iter()));
+        // Not a paste, or nothing to index: never.
+        assert!(!index_spills(16, 16, small.iter()));
+        assert!(!index_spills(5_000, 5_000, [].iter()));
+    }
+
     #[test]
     fn intervals_match_a_linear_scan() {
         let mut iv: Intervals<u32> = Intervals::default();
diff --git a/crates/calc/src/tests.rs b/crates/calc/src/tests.rs
index ac254239a..a3e7f810d 100644
--- a/crates/calc/src/tests.rs
+++ b/crates/calc/src/tests.rs
@@ -2371,3 +2371,25 @@ fn a_paste_over_small_and_tall_spills_blocks_those_it_touches() {
     assert_eq!(t.get("I1"), Value::Error(CellError::Spill));
     assert_eq!(t.num("E3"), 3.0);
 }
+
+#[test]
+fn a_big_paste_on_one_sheet_and_one_cell_over_many_spills_on_another() {
+    let mut t = T::new();
+    t.wb.sheets.push(std::sync::Arc::new(gridcraft_model::Sheet::new("Data")));
+    for i in 0..1_500u32 {
+        t.wb.sheet_mut(1).unwrap().cells.set(CellRef::new(4 * i, 0), Cell::formula(Formula::new(&format!("=SEQUENCE(3,1,{})", 10 * i))));
+    }
+    t.calc.recalc_all(&mut t.wb);
+    assert_eq!(t.wb.sheet(1).unwrap().value(CellRef::new(4 * 700 + 2, 0)), Value::Number(7002.0));
+    // 5000 cells pasted on the first sheet, one over the middle of the 701st array on the second.
+    let mut changed: Vec<(usize, CellRef)> = (0..5_000u32).map(|i| (0, CellRef::new(i / 50, i % 50))).collect();
+    changed.push((1, CellRef::new(4 * 700 + 1, 0)));
+    for k in &changed {
+        t.wb.sheet_mut(k.0).unwrap().cells.set(k.1, Cell::value(Value::Number(1.0)));
+    }
+    t.calc.cells_changed(&mut t.wb, &changed);
+    let data = t.wb.sheet(1).unwrap();
+    assert_eq!(data.value(CellRef::new(4 * 700, 0)), Value::Error(CellError::Spill));
+    assert_eq!(data.value(CellRef::new(4 * 699 + 2, 0)), Value::Number(6992.0));
+    assert_eq!(data.value(CellRef::new(4 * 701 + 2, 0)), Value::Number(7012.0));
+}