diff --git a/Tests/MeshLib/ElementUtils.h b/Tests/MeshLib/ElementUtils.h
index 5409c08504d54eb214a847f9e8eb68e06079aef6..374417fe4c2ab71438b976497fc0a997a14da733 100644
--- a/Tests/MeshLib/ElementUtils.h
+++ b/Tests/MeshLib/ElementUtils.h
@@ -61,7 +61,7 @@ inline std::unique_ptr<MeshLib::Element const> getFace(
         }
     }
 
-    OGS_FATAL("Unsupported element dimension: " + std::to_string(dim));
+    OGS_FATAL("Unsupported element dimension: {}", dim);
 }
 
 template <typename BulkElementRule>
diff --git a/Tests/MeshLib/UnitCube.h b/Tests/MeshLib/UnitCube.h
index 675527a0a0df0c77e5fede74e54983b940be49a3..bfbbdf08021ef91870b7d87e33cd5d5c70f70349 100644
--- a/Tests/MeshLib/UnitCube.h
+++ b/Tests/MeshLib/UnitCube.h
@@ -44,7 +44,7 @@ inline VTKCellType toVtk(MeshLib::CellType const cell_type)
         case CellType::PYRAMID5:
             return VTK_PYRAMID;
         default:
-            OGS_FATAL("Unsupported cell type " + CellType2String(cell_type));
+            OGS_FATAL("Unsupported cell type {}", CellType2String(cell_type));
     }
 }
 
@@ -66,10 +66,10 @@ inline void checkBounds(vtkUnstructuredGrid& grid, std::size_t const dim)
 
         if (min != -0.5 || max != 0.5)
         {
-            OGS_FATAL("Unexpected bounds for a unit cube. " +
-                      std::to_string(min) + " != -0.5 or " +
-                      std::to_string(max) + " != 0.5 for component " +
-                      std::to_string(c) + '.');
+            OGS_FATAL(
+                "Unexpected bounds for a unit cube. {} != -0.5 or {} != 0.5 "
+                "for component {}.",
+                min, max, c);
         }
     }
 }
@@ -82,9 +82,9 @@ inline void checkCellTypes(MeshLib::Mesh const& mesh,
         auto const& e = *mesh.getElement(i);
         if (e.getCellType() != cell_type)
         {
-            OGS_FATAL("Unexpected element type: " +
-                      MeshLib::CellType2String(e.getCellType()) +
-                      " != " + MeshLib::CellType2String(cell_type));
+            OGS_FATAL("Unexpected element type: {} != {}",
+                      MeshLib::CellType2String(e.getCellType()),
+                      MeshLib::CellType2String(cell_type));
         }
     }
 }
@@ -103,9 +103,9 @@ inline void checkVolume(MeshLib::Mesh const& mesh)
     if (diff > tol)
     {
         OGS_FATAL(
-            "The volume of a unit cube must be 1. Instead, the volume is " +
-            std::to_string(volume) + ". The difference is " +
-            std::to_string(diff) + " > " + std::to_string(tol) + '.');
+            "The volume of a unit cube must be 1. Instead, the volume is {}. "
+            "The difference is {:g} > {:g}",
+            volume, diff, tol);
     }
 }
 
diff --git a/Tests/NumLib/ReferenceElementUtils.cpp b/Tests/NumLib/ReferenceElementUtils.cpp
index 3a2cc47d1e308cee7f55bd1bf867ab64407ca897..5eb5e4669ff252dd1ff19ce68a92ac0e5ab4251c 100644
--- a/Tests/NumLib/ReferenceElementUtils.cpp
+++ b/Tests/NumLib/ReferenceElementUtils.cpp
@@ -53,7 +53,7 @@ getNodeCoordsOfReferenceElement(MeshLib::CellType const cell_type)
         case CellType::PYRAMID13:
             return getNodeCoordsOfReferenceElement<Pyramid13>();
         default:
-            OGS_FATAL("Unsupported cell type " + CellType2String(cell_type));
+            OGS_FATAL("Unsupported cell type {}", CellType2String(cell_type));
     }
 }
 
@@ -104,7 +104,7 @@ std::shared_ptr<MeshLib::Element const> getReferenceElement(
         case CellType::PYRAMID13:
             return create(Type<Pyramid13>{});
         default:
-            OGS_FATAL("Unsupported cell type " + CellType2String(cell_type));
+            OGS_FATAL("Unsupported cell type {}", CellType2String(cell_type));
     }
 }
 
@@ -224,6 +224,6 @@ std::vector<std::array<double, 3>> getCoordsInReferenceElementForTest(
         return coords;
     }
 
-    OGS_FATAL("Unsupported element dimension " + std::to_string(dim));
+    OGS_FATAL("Unsupported element dimension {}", dim);
 }
 }  // namespace ReferenceElementUtils
diff --git a/Tests/NumLib/ShapeFunctionUtils.h b/Tests/NumLib/ShapeFunctionUtils.h
index dd558e22f858c5340692dd096fa353895d04e62d..8f4174e56806bc942603ec7ed808c52b5b6b9f26 100644
--- a/Tests/NumLib/ShapeFunctionUtils.h
+++ b/Tests/NumLib/ShapeFunctionUtils.h
@@ -76,7 +76,8 @@ auto computeShapeMatricesOnFace(MeshLib::Element const& face,
         case CellType::PYRAMID5:
             return compute(Type<Pyramid>{});
         default:
-            OGS_FATAL("Unsupported cell type " + CellType2String(cell_type));
+            OGS_FATAL("Unsupported cell type {}",
+                      MeshLib::CellType2String(cell_type));
     }
 }
 
@@ -132,7 +133,8 @@ auto interpolateNodeCoordinatesFace(MeshLib::Element const& face,
         case CellType::PYRAMID5:
             return interpolate(Type<Pyramid>{});
         default:
-            OGS_FATAL("Unsupported cell type " + CellType2String(cell_type));
+            OGS_FATAL("Unsupported cell type {}",
+                      MeshLib::CellType2String(cell_type));
     }
 }
 }  // namespace ShapeFunctionUtils