diff --git a/NanoVNASaver/Charts/LogMag.py b/NanoVNASaver/Charts/LogMag.py
index c0852bd..32efad7 100644
--- a/NanoVNASaver/Charts/LogMag.py
+++ b/NanoVNASaver/Charts/LogMag.py
@@ -23,35 +23,29 @@ from typing import List
 
 from PyQt5 import QtGui
 
-from NanoVNASaver.RFTools import Datapoint
 from NanoVNASaver.Charts.Chart import Chart
 from NanoVNASaver.Charts.Frequency import FrequencyChart
+from NanoVNASaver.RFTools import Datapoint
+from NanoVNASaver.SITools import log_floor_125
 
 logger = logging.getLogger(__name__)
 
 
 @dataclass
 class TickVal:
-    count: int
-    first: float
-    step: float
+    count: int = 0
+    first: float = 0.0
+    step: float = 0.0
 
 
 def span2ticks(span: float, min_val: float) -> TickVal:
-    for spn, dbs in ((50.0, 10.0),
-                     (20.0, 5.0),
-                     (10.0, 2.0),
-                     (5.0, 1.0),
-                     (2.0, 0.5),
-                     (1.0, 0.2),
-                     (0.0, 0.1)):
-        if span >= spn:
-            count = math.floor(span / dbs)
-            first = math.ceil(min_val / dbs) * dbs
-            step = dbs
-            if first == min_val:
-                first += dbs
-            break
+    logger.debug("span2ticks(%s, %s)", span, min_val)
+    span = abs(span)
+    step = log_floor_125(span / 5)
+    count = math.floor(span / step)
+    first = math.ceil(min_val / step) * step
+    if first == min_val:
+        first += step
     return TickVal(count, first, step)
 
 
diff --git a/NanoVNASaver/SITools.py b/NanoVNASaver/SITools.py
index c3262b1..dcc019b 100644
--- a/NanoVNASaver/SITools.py
+++ b/NanoVNASaver/SITools.py
@@ -41,6 +41,16 @@ def round_floor(value: Real, digits: int = 0) -> Real:
     return factor * math.floor(value / factor)
 
 
+def log_floor_125(x: float) -> float:
+    log_base = 10**(math.floor(math.log10(x)))
+    log_factor = x / log_base
+    if log_factor >= 5:
+        return 5 * log_base
+    if log_factor >= 2:
+        return 2 * log_base
+    return log_base
+
+
 class Format(NamedTuple):
     max_nr_digits: int = 6
     fix_decimals: bool = False
diff --git a/test/test_sitools.py b/test/test_sitools.py
index 6b718ff..ca324b1 100644
--- a/test/test_sitools.py
+++ b/test/test_sitools.py
@@ -21,7 +21,9 @@ from math import inf, nan
 from decimal import Decimal  # Needed for test_representation()
 
 # Import targets to be tested
-from NanoVNASaver.SITools import Format, Value, round_floor, round_ceil
+from NanoVNASaver.SITools import (
+    Format, Value, round_floor, round_ceil, log_floor_125
+)
 
 F_DEFAULT = Format()
 
@@ -43,7 +45,7 @@ F_WITH_SPACE = Format(space_str=" ")
 F_WITH_UNDERSCORE = Format(space_str="_")
 
 
-class TestTSIToolsValue(unittest.TestCase):
+class TestSIToolsValue(unittest.TestCase):
 
     def test_format_assertions(self):
         self.assertRaises(AssertionError, Value, fmt=F_ASSERT_DIGITS_0)
@@ -167,3 +169,20 @@ class TestTSIToolsValue(unittest.TestCase):
 # TODO: test F_DIGITS_31
 #            F_WITH_SPACE
 #            F_WITH_UNDERSCORE
+
+
+class TestSIToolsFunctions(unittest.TestCase):
+
+    def test_log_floor_125(self):
+        self.assertEqual(log_floor_125(1), 1)
+        self.assertEqual(log_floor_125(2), 2)
+        self.assertEqual(log_floor_125(5), 5)
+        self.assertEqual(log_floor_125(1000), 1000)
+        self.assertEqual(log_floor_125(2000), 2000)
+        self.assertEqual(log_floor_125(5000), 5000)
+        self.assertEqual(log_floor_125(.01), .01)
+        self.assertEqual(log_floor_125(.02), .02)
+        self.assertEqual(log_floor_125(.05), .05)
+        self.assertEqual(log_floor_125(1.9), 1)
+        self.assertEqual(log_floor_125(4.5), 2)
+        self.assertEqual(log_floor_125(9.9), 5)