diff --git a/cdm/core/src/main/java/ucar/nc2/ft2/coverage/writer/CFGridCoverageWriter.java b/cdm/core/src/main/java/ucar/nc2/ft2/coverage/writer/CFGridCoverageWriter.java
index 1e9063e8ce..049fb4ee65 100644
--- a/cdm/core/src/main/java/ucar/nc2/ft2/coverage/writer/CFGridCoverageWriter.java
+++ b/cdm/core/src/main/java/ucar/nc2/ft2/coverage/writer/CFGridCoverageWriter.java
@@ -155,14 +155,14 @@ private Result writeFile(CoverageCollection gdsOrg, List<String> gridNames, Subs
     }
     addCFAnnotations(subsetDataset, rootGroup, shouldAddLatLon2D);
 
+    // test if its too large
+    long totalSizeOfVars = writer.calcSize();
+    if (maxBytes > 0 && totalSizeOfVars > maxBytes) {
+      return Result.create(totalSizeOfVars, false, TOO_LARGE_MESSAGE);
+    }
+
     // Actually create file and write variable data to it.
     try (NetcdfFormatWriter ncwriter = writer.build()) {
-      // test if its too large
-      long totalSizeOfVars = ncwriter.calcSize();
-      if (maxBytes > 0 && totalSizeOfVars > maxBytes) {
-        return Result.create(totalSizeOfVars, false, TOO_LARGE_MESSAGE);
-      }
-
       writeCoordinateData(subsetDataset, ncwriter);
       writeCoverageData(gdsOrg, subsetParams, subsetDataset, ncwriter);
 
diff --git a/cdm/core/src/main/java/ucar/nc2/write/NetcdfFormatWriter.java b/cdm/core/src/main/java/ucar/nc2/write/NetcdfFormatWriter.java
index d766ea185b..3d968eceb5 100644
--- a/cdm/core/src/main/java/ucar/nc2/write/NetcdfFormatWriter.java
+++ b/cdm/core/src/main/java/ucar/nc2/write/NetcdfFormatWriter.java
@@ -246,6 +246,22 @@ public Structure.Builder addStructure(String shortName, String dimString) {
       return vb;
     }
 
+    public long calcSize() {
+      return calcSize(this.rootGroup);
+    }
+
+    // Note that we have enough info to try to estimate effects of compression, if its a Netcdf4 file.
+    private long calcSize(Group.Builder group) {
+      long totalSizeOfVars = 0;
+      for (Variable.Builder var : this.rootGroup.vbuilders) {
+        totalSizeOfVars += Dimensions.getSize(var.getDimensions()) * var.getElementSize();
+      }
+      for (Group.Builder nested : group.gbuilders) {
+        totalSizeOfVars += calcSize(nested);
+      }
+      return totalSizeOfVars;
+    }
+
     /** Once this is called, do not use the Builder again. */
     public NetcdfFormatWriter build() throws IOException {
       return new NetcdfFormatWriter(this);