diff --git a/crates/forms/src/af/calc.rs b/crates/forms/src/af/calc.rs
index 1c3724ef4a..66738246c2 100644
--- a/crates/forms/src/af/calc.rs
+++ b/crates/forms/src/af/calc.rs
@@ -612,6 +612,9 @@ pub(super) fn eval_notation(src: &str, value_of: &dyn Fn(&str) -> f64) -> Option
     (i == toks.len() && v.is_finite()).then_some(v)
 }
 
+/// Most field values one `AFSimple_Calculate` list may expand to (a form has some thousands).
+const MAX_EXPANDED_VALUES: usize = 1 << 20;
+
 /// The Calculate event: the new value, given the current values of the other fields
 /// (`values(name)` returns every field named `name` or below it, for groups).
 pub fn calculate(c: &Calculate, values: &dyn Fn(&str) -> Vec<String>) -> Option<String> {
@@ -623,20 +626,21 @@ pub fn calculate(c: &Calculate, values: &dyn Fn(&str) -> Vec<String>) -> Option<
         // doesn't count it as 1. (The form's `values` leaves out a field that has no value.)
         Calculate::Simple { op: CalcOp::Product, fields } if fields.iter().all(|f| values(f).is_empty()) => Some("0".into()),
         Calculate::Simple { op, fields } => {
-            let xs: Vec<f64> = fields.iter().flat_map(|f| values(f)).map(|v| num(&v)).collect();
+            // The values are folded as they are read, never collected: a group name listed many
+            // times would otherwise be held once per listing. A list that expands past
+            // MAX_EXPANDED_VALUES is not calculated.
+            let mut n = 0usize;
+            let xs = fields.iter().flat_map(|f| values(f)).map(|v| num(&v)).inspect(|_| n += 1).take(MAX_EXPANDED_VALUES + 1);
             let v = match op {
-                CalcOp::Sum => xs.iter().sum(),
-                CalcOp::Product => xs.iter().product(),
-                CalcOp::Average => {
-                    if xs.is_empty() {
-                        0.0
-                    } else {
-                        xs.iter().sum::<f64>() / xs.len() as f64
-                    }
-                }
-                CalcOp::Minimum => xs.iter().copied().fold(f64::INFINITY, f64::min),
-                CalcOp::Maximum => xs.iter().copied().fold(f64::NEG_INFINITY, f64::max),
+                CalcOp::Sum | CalcOp::Average => xs.sum(),
+                CalcOp::Product => xs.product(),
+                CalcOp::Minimum => xs.fold(f64::INFINITY, f64::min),
+                CalcOp::Maximum => xs.fold(f64::NEG_INFINITY, f64::max),
             };
+            if n > MAX_EXPANDED_VALUES {
+                return None;
+            }
+            let v = if *op == CalcOp::Average { if n == 0 { 0.0 } else { v / n as f64 } } else { v };
             Some(if v.is_finite() { js_number(v) } else { "0".into() })
         }
         Calculate::Notation(expr) => {
diff --git a/crates/forms/src/af/tests.rs b/crates/forms/src/af/tests.rs
index 3c6dab1bef..b4b6452a3c 100644
--- a/crates/forms/src/af/tests.rs
+++ b/crates/forms/src/af/tests.rs
@@ -486,3 +486,23 @@ fn destructured_for_of_targets_keep_regex_iterables_opaque() {
         }
     }
 }
+
+#[test]
+fn simple_calculations_fold_a_repeated_group_without_collecting_it() {
+    let group = |_: &str| vec!["2".to_string(); 3];
+    let list = |op, n: usize| Calculate::Simple { op, fields: vec!["g".to_string(); n] };
+    assert_eq!(calculate(&list(CalcOp::Sum, 4), &group).as_deref(), Some("24"));
+    assert_eq!(calculate(&list(CalcOp::Average, 4), &group).as_deref(), Some("2"));
+    assert_eq!(calculate(&list(CalcOp::Product, 2), &group).as_deref(), Some("64"));
+    assert_eq!(calculate(&list(CalcOp::Maximum, 2), &group).as_deref(), Some("2"));
+    assert_eq!(calculate(&list(CalcOp::Sum, 0), &group).as_deref(), Some("0"));
+}
+
+#[test]
+fn a_simple_calculation_that_expands_past_the_allowance_is_not_calculated() {
+    let thousand = |_: &str| vec!["1".to_string(); 1000];
+    let within = Calculate::Simple { op: CalcOp::Sum, fields: vec!["g".to_string(); 1000] };
+    let past = Calculate::Simple { op: CalcOp::Sum, fields: vec!["g".to_string(); 2000] };
+    assert_eq!(calculate(&within, &thousand).as_deref(), Some("1000000"));
+    assert_eq!(calculate(&past, &thousand), None);
+}