diff --git a/Mathlib.lean b/Mathlib.lean
index bb572a5a061e4f..a9f7cb067dec9b 100644
--- a/Mathlib.lean
+++ b/Mathlib.lean
@@ -6010,6 +6010,7 @@ public import Mathlib.Order.InitialSeg
 public import Mathlib.Order.Interval.Basic
 public import Mathlib.Order.Interval.Finset.Basic
 public import Mathlib.Order.Interval.Finset.Box
+public import Mathlib.Order.Interval.Finset.Card
 public import Mathlib.Order.Interval.Finset.Defs
 public import Mathlib.Order.Interval.Finset.DenselyOrdered
 public import Mathlib.Order.Interval.Finset.Fin
diff --git a/Mathlib/Order/Interval/Finset/Card.lean b/Mathlib/Order/Interval/Finset/Card.lean
new file mode 100644
index 00000000000000..6cbf599f3f1246
--- /dev/null
+++ b/Mathlib/Order/Interval/Finset/Card.lean
@@ -0,0 +1,114 @@
+/-
+Copyright (c) 2026 Snir Broshi. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Snir Broshi
+-/
+module
+
+public import Mathlib.Data.Set.Card
+public import Mathlib.Order.Interval.Finset.Defs
+
+/-!
+# Cardinalities of intervals
+
+Cardinalities of intervals can be computed using finsets in locally finite orders.
+-/
+
+public section
+
+open Finset
+
+variable {α : Type*}
+
+section Preorder
+
+variable [Preorder α]
+
+section LocallyFiniteOrder
+
+variable [LocallyFiniteOrder α]
+
+@[to_dual self]
+theorem Set.cardinalMk_Icc (a b : α) : Cardinal.mk (Set.Icc a b) = #(Finset.Icc a b) := by
+  simp
+
+@[to_dual self]
+theorem Set.encard_Icc (a b : α) : (Set.Icc a b).encard = #(Finset.Icc a b) := by
+  simp [← Finset.coe_Icc]
+
+@[to_dual self]
+theorem Set.ncard_Icc (a b : α) : (Set.Icc a b).ncard = #(Finset.Icc a b) := by
+  simp [← Finset.coe_Icc]
+
+@[to_dual (reorder := a b)]
+theorem Set.cardinalMk_Ico (a b : α) : Cardinal.mk (Set.Ico a b) = #(Finset.Ico a b) := by
+  simp
+
+@[to_dual (reorder := a b)]
+theorem Set.encard_Ico (a b : α) : (Set.Ico a b).encard = #(Finset.Ico a b) := by
+  simp [← Finset.coe_Ico]
+
+@[to_dual (reorder := a b)]
+theorem Set.ncard_Ico (a b : α) : (Set.Ico a b).ncard = #(Finset.Ico a b) := by
+  simp [← Finset.coe_Ico]
+
+@[to_dual self]
+theorem Set.cardinalMk_Ioo (a b : α) : Cardinal.mk (Set.Ioo a b) = #(Finset.Ioo a b) := by
+  simp
+
+@[to_dual self]
+theorem Set.encard_Ioo (a b : α) : (Set.Ioo a b).encard = #(Finset.Ioo a b) := by
+  simp [← Finset.coe_Ioo]
+
+@[to_dual self]
+theorem Set.ncard_Ioo (a b : α) : (Set.Ioo a b).ncard = #(Finset.Ioo a b) := by
+  simp [← Finset.coe_Ioo]
+
+end LocallyFiniteOrder
+
+section LocallyFiniteOrderTop
+
+variable [LocallyFiniteOrderTop α]
+
+@[to_dual]
+theorem Set.cardinalMk_Ici (a : α) : Cardinal.mk (Set.Ici a) = #(Finset.Ici a) := by
+  simp
+
+@[to_dual]
+theorem Set.encard_Ici (a : α) : (Set.Ici a).encard = #(Finset.Ici a) := by
+  simp [← Finset.coe_Ici]
+
+@[to_dual]
+theorem Set.ncard_Ici (a : α) : (Set.Ici a).ncard = #(Finset.Ici a) := by
+  simp [← Finset.coe_Ici]
+
+@[to_dual]
+theorem Set.cardinalMk_Ioi (a : α) : Cardinal.mk (Set.Ioi a) = #(Finset.Ioi a) := by
+  simp
+
+@[to_dual]
+theorem Set.encard_Ioi (a : α) : (Set.Ioi a).encard = #(Finset.Ioi a) := by
+  simp [← Finset.coe_Ioi]
+
+@[to_dual]
+theorem Set.ncard_Ioi (a : α) : (Set.Ioi a).ncard = #(Finset.Ioi a) := by
+  simp [← Finset.coe_Ioi]
+
+end LocallyFiniteOrderTop
+
+end Preorder
+
+section Lattice
+
+variable [Lattice α] [LocallyFiniteOrder α]
+
+theorem Set.cardinalMk_uIcc (a b : α) : Cardinal.mk (Set.uIcc a b) = #(Finset.uIcc a b) := by
+  simp
+
+theorem Set.encard_uIcc (a b : α) : (Set.uIcc a b).encard = #(Finset.uIcc a b) := by
+  simp [← Finset.coe_uIcc]
+
+theorem Set.ncard_uIcc (a b : α) : (Set.uIcc a b).ncard = #(Finset.uIcc a b) := by
+  simp [← Finset.coe_uIcc]
+
+end Lattice